Checked about:config but setting was already true. My Windows Manager theme is Peppermix-Dark-7, Widget is Pepirus and Icon Theme is Pepirus-Dark. However I tried Pepirus and Peppermix-Dark 7-Red for Windows Manager theme plus various Widget/Icon themes and no effect on Firefox tabs. The other difference I notice in Firefox is highlighting text in search boxes is now red in Default setting but remains blue (for me!) in either Light or Dark theme. However I don't think it worth pursuing unless others complain because I note that with my Peppermix-Dark-7 theme the Firefox window border is blue, so blue burst on Firefox tab kind of compliments it. Anyway its all very minor as Peppermint 8 Respin is the best distro out there. Thank you all for a wonderful system and look forward to P9, although I'm not sure what further improvements you can make except keeping up with latest Ubuntu LTS.
EDIT: Just a thought - my Firefox is synced with other distros plus Windows 7 all running the Dark theme. However I note that the Default theme on these other distros remains blue even as Peppermint Default stays red, so maybe no effect?