I have found one it won't and that is Browsec VPN but other than that it seems fast and it works with the Forums which is a must 
I stand corrected on the plugins thing.

The web browser portion is based on Gecko, just like Firefox is. (though, it probably can't save you money on car insurance

) So, pretty much any site that's designed to work with Firefox, which is the vast majority of them, should work with SeaMonkey also. (operative word: should)