My Firefox memory bloat was mostly from All-in-One Gestures
It's time for an update to my prior Firefox situation (one, two). After some experimentation it's become clear that most of my Firefox problems with constant memory growth and zombie compartments were due to my use of All-in-One Gestures (as I kind of suspected it might be). I've switched to FireGestures instead (initially as an experiment and now full time on all of my various Firefox instances on various different machines) and things have been much better; there are no zombie compartments at all and memory growth seems to have dropped significantly (although it's not clear yet if it's completely gone). And I haven't run into any problems or bugs this time around; everything has just worked the way I expected.
(A-i-O doesn't seem to have been the only problem I had; for example, it seems to be a bad idea to leave a tab or window sitting around with an embedded Youtube video. It's also not clear if Firefox Nightly behaves well for me in general because I haven't been able to leave it running for multiple days yet.)
In addition to less memory usage, FireGestures also seems to simply be more responsive and snappy than A-i-O. It certainly has more useful features, including the ability to add gestures without needing to hack the source code, a library of existing additional gestures (including the one that I wanted), and the ability to 'back up' and 'restore' your settings (which for me really means the ability to easily synchronize my gestures between multiple Firefox instances).
(See FireGesture's homepage for more information on all of this.)
The one drawback of FireGestures is that it doesn't work in Firefox 3.6; my laptop is still running Fedora 14 with this Firefox release (because that's the last one with Gnome 2 instead of Gnome 3). I don't consider this a real drawback, but you may.
PS: people migrating from All-in-One Gestures to FireGestures might want to use Down-Right-Down to call up the A-i-O information display that shows all of your gestures and then save it (as an HTML page, which is what it is). You can then conveniently look at it later when you're using FireGestures.
(I am far too impatient to try to retrain years of reflexes to use the native FireGestures gestures for various actions; I just ruthlessly rewrote them to be the A-i-O gestures I'm used to.)