OK. Thanks for the fast response. The first case (i.e., the Browsers category) is simpler, so I'll go back to that, as I'm having the same problem there.
In Peppermint 10, I've added a Browsers category among the other top-level categories of Menu (e.g., Accessories, Multimedia). I added that category by copying the lines for another top-level category in ~/.config/menus/xfce-applications.menu and revising them for Browsers, like this:
<Menu>
<Name>Browsers</Name>
<Directory>xfce-browsers.directory</Directory>
<DirectoryDir>/home/ray/.local/share/desktop-directories</DirectoryDir>
<Include>
<Category>Browsers</Category>
</Include>
<Layout>
<Merge type="menus" />
<Merge type="files" />
</Layout>
</Menu>
I also added <Menuname>Browsers</Menuname> among the other Menuname entries near the bottom of xfce-applications.menu.
Next, I copied one of the other directory files in ~/.local/share/desktop-directories, named the copy xfce-browsers.directory, and revised it thus:
[Desktop Entry]
Version=1.1
Type=Directory
Name=Browsers
Comment=Browser-related applications
Icon=applications-development-web
Then I copied ice.desktop from the systemwide launcher pool at /usr/share/applications to ~/.local/share/applications, and edited it by simply adding "Browsers;" to its Categories line. I saved that, restarted MenuLibre, and observed that both Menu and MenuLibre were displaying a Browsers category with an Ice launcher. So far, so good.
Now I sought to add a second program's launcher to the Browsers category. The program: Firefox Send, whose launcher (firefoxsend.desktop) already existed in ~/.local/share/applications. I merely added "Browsers;" to its Categories line and saved the change. Now, even after a reboot, Ice was gone from the Browsers category, and (therefore) Browsers no longer appeared in Menu.
Questions at this point: did I do it right, why didn't Ice and Firefox Send appear in Browsers, and is there a better place to get icons?
The text of xfce-applications.menu at present:
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE Menu
PUBLIC '-//freedesktop//DTD Menu 1.0//EN'
'http://standards.freedesktop.org/menu-spec/menu-1.0.dtd'>
<Menu>
<Name>Xfce</Name>
<DefaultAppDirs />
<DefaultDirectoryDirs />
<DefaultMergeDirs />
<Include>
<Category>X-Xfce-Toplevel</Category>
</Include>
<MergeFile type="parent">/etc/xdg/menus/xfce-applications.menu</MergeFile>
<Menu>
<Name>Settings</Name>
<Directory>xfce-settings.directory</Directory>
<DirectoryDir>/home/ray/.local/share/desktop-directories</DirectoryDir>
<Menu>
<Name>Screensavers</Name>
<Directory>xfce-screensavers.directory</Directory>
<Layout>
<Merge type="menus" />
<Merge type="files" />
</Layout>
</Menu>
<Layout>
<Merge type="menus" />
<Separator />
<Filename>software-properties-drivers.desktop</Filename>
<Filename>advert-block.desktop</Filename>
<Filename>arandr.desktop</Filename>
<Filename>blueman-manager.desktop</Filename>
<Filename>lxappearance.desktop</Filename>
<Filename>lxsession-default-apps.desktop</Filename>
<Filename>org.gnome.DiskUtility.desktop</Filename>
<Filename>xfce-display-settings.desktop</Filename>
<Filename>peppermint-firefox-themer.desktop</Filename>
<Filename>gufw.desktop</Filename>
<Filename>peppermint-font-dpi-settings.desktop</Filename>
<Filename>im-config.desktop</Filename>
<Filename>gnome-language-selector.desktop</Filename>
<Filename>light-locker-settings.desktop</Filename>
<Filename>lightdm-settings.desktop</Filename>
<Filename>menulibre.desktop</Filename>
<Filename>network.desktop</Filename>
<Filename>nm-connection-editor.desktop</Filename>
<Filename>numix-folders.desktop</Filename>
<Filename>peppermint-control-center.desktop</Filename>
<Filename>peppermint-proxy-configuration.desktop</Filename>
<Filename>peppermint-settings-panel.desktop</Filename>
<Filename>xfce4-power-manager-settings.desktop</Filename>
<Filename>system-config-printer.desktop</Filename>
<Filename>system-config-samba.desktop</Filename>
<Menuname>Screensavers</Menuname>
<Filename>shares.desktop</Filename>
<Filename>mintinstall.desktop</Filename>
<Filename>software-properties-gtk.desktop</Filename>
<Filename>peppermint-thunderbird-themer.desktop</Filename>
<Filename>time.desktop</Filename>
<Filename>mintUpdate.desktop</Filename>
<Filename>users.desktop</Filename>
<Filename>xfpanel-switch.desktop</Filename>
<Merge type="files" />
</Layout>
</Menu>
<Menu>
<Name>Accessories</Name>
<Directory>xfce-accessories.directory</Directory>
<DirectoryDir>/home/ray/.local/share/desktop-directories</DirectoryDir>
<Layout>
<Merge type="menus" />
<Filename>xfce4-appfinder.desktop</Filename>
<Filename>file-roller.desktop</Filename>
<Filename>org.gnome.FileRoller.desktop</Filename>
<Filename>mate-calc.desktop</Filename>
<Filename>gucharmap.desktop</Filename>
<Filename>compton.desktop</Filename>
<Filename>org.gnome.DiskUtility.desktop</Filename>
<Filename>xreader.desktop</Filename>
<Filename>nemo.desktop</Filename>
<Filename>yad-icon-browser.desktop</Filename>
<Filename>menulibre.desktop</Filename>
<Filename>xfce4-run.desktop</Filename>
<Filename>xfce4-screenshooter.desktop</Filename>
<Filename>peppermint-scrot-helper.desktop</Filename>
<Filename>gnome-search-tool.desktop</Filename>
<Filename>xfce4-taskmanager.desktop</Filename>
<Filename>sakura.desktop</Filename>
<Filename>xed.desktop</Filename>
<Filename>nitrogen.desktop</Filename>
<Merge type="files" />
</Layout>
</Menu>
<Menu>
<Name>Development</Name>
<Directory>xfce-development.directory</Directory>
<DirectoryDir>/home/ray/.local/share/desktop-directories</DirectoryDir>
<Layout>
<Merge type="menus" />
<Filename>python2.7.desktop</Filename>
<Filename>python3.6.desktop</Filename>
<Merge type="files" />
</Layout>
</Menu>
<Menu>
<Name>Education</Name>
<Directory>xfce-education.directory</Directory>
<DirectoryDir>/home/ray/.local/share/desktop-directories</DirectoryDir>
<Layout>
<Merge type="menus" />
<Merge type="files" />
</Layout>
</Menu>
<Menu>
<Name>Games</Name>
<Directory>xfce-games.directory</Directory>
<DirectoryDir>/home/ray/.local/share/desktop-directories</DirectoryDir>
<Layout>
<Merge type="menus" />
<Filename>betterthanchess.desktop</Filename>
<Filename>entanglement.desktop</Filename>
<Filename>firstpersontetris.desktop</Filename>
<Filename>mahjong.desktop</Filename>
<Filename>solitaire.desktop</Filename>
<Filename>spidersolitaire.desktop</Filename>
<Merge type="files" />
</Layout>
</Menu>
<Menu>
<Name>Graphics</Name>
<Directory>xfce-graphics.directory</Directory>
<DirectoryDir>/home/ray/.local/share/desktop-directories</DirectoryDir>
<Layout>
<Merge type="menus" />
<Filename>xviewer.desktop</Filename>
<Filename>editorbypixlr.desktop</Filename>
<Filename>expressbypixlr.desktop</Filename>
<Filename>simple-scan.desktop</Filename>
<Merge type="files" />
</Layout>
</Menu>
<Menu>
<Name>Browsers</Name>
<Directory>xfce-browsers.directory</Directory>
<DirectoryDir>/home/ray/.local/share/desktop-directories</DirectoryDir>
<Layout>
<Merge type="menus" />
<Filename>ice.desktop</Filename>
<Merge type="files" />
</Layout>
</Menu>
<Menu>
<Name>Network</Name>
<Directory>xfce-network.directory</Directory>
<DirectoryDir>/home/ray/.local/share/desktop-directories</DirectoryDir>
<Layout>
<Merge type="menus" />
<Filename>googledrive.desktop</Filename>
<Filename>transmission-gtk.desktop</Filename>
<Filename>dropbox.desktop</Filename>
<Filename>firefoxsend.desktop</Filename>
<Filename>firefox.desktop</Filename>
<Filename>ice.desktop</Filename>
<Filename>onlineuserguide.desktop</Filename>
<Filename>peppermintforum.desktop</Filename>
<Merge type="files" />
</Layout>
</Menu>
<Menu>
<Name>Multimedia</Name>
<Directory>xfce-multimedia.directory</Directory>
<DirectoryDir>/home/ray/.local/share/desktop-directories</DirectoryDir>
<Layout>
<Merge type="menus" />
<Filename>guvcview.desktop</Filename>
<Filename>xplayer.desktop</Filename>
<Filename>pulseaudio-equalizer.desktop</Filename>
<Filename>pavucontrol.desktop</Filename>
<Merge type="files" />
</Layout>
</Menu>
<Menu>
<Name>Office</Name>
<Directory>xfce-office.directory</Directory>
<DirectoryDir>/home/ray/.local/share/desktop-directories</DirectoryDir>
<Layout>
<Merge type="menus" />
<Filename>gmail.desktop</Filename>
<Filename>googlecalendar.desktop</Filename>
<Filename>microsoftexcelonline.desktop</Filename>
<Filename>microsoftonenoteonline.desktop</Filename>
<Filename>microsoftpowerpointonline.desktop</Filename>
<Filename>microsoftwordonline.desktop</Filename>
<Merge type="files" />
</Layout>
</Menu>
<Menu>
<Name>Other</Name>
<Directory>xfce-other.directory</Directory>
<DirectoryDir>/home/ray/.local/share/desktop-directories</DirectoryDir>
<Include>
<Filename>gcr-prompter.desktop</Filename>
</Include>
<Include>
<Filename>nemo-autorun-software.desktop</Filename>
</Include>
<Include>
<Filename>panel-desktop-handler.desktop</Filename>
</Include>
<Include>
<Filename>gnome-disk-image-mounter.desktop</Filename>
</Include>
<Include>
<Filename>gnome-disk-image-writer.desktop</Filename>
</Include>
<Include>
<Filename>peppermint-logout.desktop</Filename>
</Include>
<Include>
<Filename>snap-handle-link.desktop</Filename>
</Include>
<Include>
<Filename>metacity.desktop</Filename>
</Include>
<Include>
<Filename>nm-applet.desktop</Filename>
</Include>
<Include>
<Filename>notification-daemon.desktop</Filename>
</Include>
<Include>
<Filename>xdg-desktop-portal-gtk.desktop</Filename>
</Include>
<Include>
<Filename>mintinstall-fp-handler.desktop</Filename>
</Include>
<Include>
<Filename>gcr-viewer.desktop</Filename>
</Include>
<Layout>
<Merge type="menus" />
<Filename>firefoxsend.desktop</Filename>
<Filename>ice.desktop</Filename>
<Filename>gcr-prompter.desktop</Filename>
<Filename>nemo-autorun-software.desktop</Filename>
<Filename>panel-desktop-handler.desktop</Filename>
<Filename>gnome-disk-image-mounter.desktop</Filename>
<Filename>gnome-disk-image-writer.desktop</Filename>
<Filename>peppermint-logout.desktop</Filename>
<Filename>snap-handle-link.desktop</Filename>
<Filename>metacity.desktop</Filename>
<Filename>nm-applet.desktop</Filename>
<Filename>notification-daemon.desktop</Filename>
<Filename>xdg-desktop-portal-gtk.desktop</Filename>
<Filename>mintinstall-fp-handler.desktop</Filename>
<Filename>gcr-viewer.desktop</Filename>
<Merge type="files" />
</Layout>
</Menu>
<Menu>
<Name>System</Name>
<Directory>xfce-system.directory</Directory>
<DirectoryDir>/home/ray/.local/share/desktop-directories</DirectoryDir>
<Layout>
<Merge type="menus" />
<Filename>org.gnome.Software.Editor.desktop</Filename>
<Filename>ca.desrt.dconf-editor.desktop</Filename>
<Filename>gdebi.desktop</Filename>
<Filename>htop.desktop</Filename>
<Filename>lightdm-settings.desktop</Filename>
<Filename>network.desktop</Filename>
<Filename>system-config-printer.desktop</Filename>
<Filename>hplj1020.desktop</Filename>
<Filename>gksu.desktop</Filename>
<Filename>system-config-samba.desktop</Filename>
<Filename>peppermint-screenlock.desktop</Filename>
<Filename>shares.desktop</Filename>
<Filename>org.gnome.Software.desktop</Filename>
<Filename>gnome-software-local-file.desktop</Filename>
<Filename>mintinstall.desktop</Filename>
<Filename>synaptic.desktop</Filename>
<Filename>xfce4-taskmanager.desktop</Filename>
<Filename>time.desktop</Filename>
<Filename>mintUpdate.desktop</Filename>
<Filename>mintstick.desktop</Filename>
<Filename>mintstick-format.desktop</Filename>
<Filename>users.desktop</Filename>
<Filename>debian-uxterm.desktop</Filename>
<Filename>debian-xterm.desktop</Filename>
<Merge type="files" />
</Layout>
</Menu>
<Layout>
<Filename>xfce4-run.desktop</Filename>
<Separator />
<Separator />
<Menuname>Settings</Menuname>
<Separator />
<Menuname>Accessories</Menuname>
<Menuname>Development</Menuname>
<Menuname>Education</Menuname>
<Menuname>Games</Menuname>
<Menuname>Graphics</Menuname>
<Menuname>Browsers</Menuname>
<Menuname>Network</Menuname>
<Menuname>Multimedia</Menuname>
<Menuname>Office</Menuname>
<Menuname>Other</Menuname>
<Menuname>System</Menuname>
<Separator />
</Layout>
</Menu>