I doubt it. Each machine spec would be different and each extension have different purposes.
Ex. with the Evernote icon, you still have to factor in having Evernote opened as part of consuming your memory.
There's also extensions like Taboo where the more you put in, the more memory Firefox takes up.
The best tip I can think of for someone who's really bothered by Firefox's slowness is to multi-browse with Opera and add only non-Opera giving extensions to Firefox and use Opera's open page in Firefox's capability to supplement sites that you require extensions for.
Either that or upgrade the machine though I'm using more than 14 extensions and FF isn't drastically slowing me down. The back button though is no problem if you multi-browse with Opera although it doesn't bother me that much when using FF.