While I did get around to my one-extention-at-a-time testing of Firefox slowness I still haven’t got around to writing more about it. My, rough, observations are the following:

1. The longer Firefox is running the slower everything gets. As much as I’d love to turn my computer off every night and start afresh every morning the fact is that the best I can do really is put it into suspend, without wasting time every morning (I work at home and maintain a lot of state on my computer.) Another solution would be to stop-start firefox every now and then since it’ll reload my last set of open tabs/windows on starting, I really shouldn’t have to do that though, and it leads to point 2…
2. More tabs seems to mean more slowness. I tend to be tab-happy, like my 20-desktop(+tabs) spaces of state I find keeping stuff open in the browser to be simpler than worrying about “saving tabsets” and bookmarking everything (and trying to work out one end of the “history” from the other is just not going to work.) To this end I tend to have 2 or three browser windows open at a time with up to three rows of tabs in each. The slowness isn’t Firefox hitting swap so must be something else (Firefox memory use seems to grow linearly with number of tabs open, fair enough I guess since memory is cheap these days.) Maybe a plugin or extension, I haven’t tried the incremental-plugin/extension-addition + tab-growth experiment.
3. The “SwitchProxy Tool” extension does seem to cause slowness. I’m confounded as to why it should be so!
4. The “Google Browser Sync” extension also causes slowness. Again, I have no idea why, but it does a pretty complicated job.

That’s all I have to report. And AFAIC “problem solved!” “How so?!” You ask? Simple: I’ve mostly stopped using Firefox. Over the last few weeks I’ve been making a gradual switch to Opera. Oh no! It’s closed-source evilware! shrug It is:

1. Faster (oh, so much faster!), more responsive when I have 50 tabs open in three windows.
2. No longer encumbered with that annoying ad panel.
3. Now equipped (in the current beta) with native browser-sync functionality. This is nice since Google Browser Sync has become, for me, the most important Firefox feature. That said, I don’t know how “trusted” the Opera browser sync is and it certainly doesn’t seem to be as comprehensive as the Google offering. I expect Opera sync will be expanded in time.
4. No more of a memory hog than Firefox (as far as I can see, but I haven’t tested this exhaustively.)
5. Binary packaged for every distro I use and there’s an apt repository (though without Ubuntu dists, my sources.list line on Ubuntu gutsy is for Debian testing.)

On the downsides:

1. Has occasional problems with Flash, which require a browser restart but luckily Opera comes back up with all my windows and tabs rather quickly. Yes, in an ideal world there’d be no flash, wank, wank. Flash is a “fact of life” as far as the web goes these days, and IMO it gives us some useful things that would XHTML/CSS can’t. This particular problem is my most pressing issue with Opera at the moment.
2. No less of a memory hog than Firefox. This is based on occasional glances at top while running both, not any proper testing!
3. Much less of a CPU hog than Firefox but still pretty bad at idle sometimes. However this is mostly the fault of websites to some degree, it just seems that Opera keeps a tighter hold on not letting a site’s javascript get out of hand. For example, having smh.com.au open in Firefox really bogs it down, while Opera remains responsive (albeit at a constant ~10% CPU usage on my desktop.) It should be noted that the smh.com.au site runs some particularly disgusting javascript.
4. Hitting sites that “don’t work in Opera” is more common than “don’t work in Firefox”, but the Web’s become pretty good in this regard these days so this is fairly rare.

It should be noted that I’m using a beta version of Opera (on Ubuntu Gutsy.)

There are things I haven’t tried or worked out with Opera yet. Namely:

1. What’s on offer as far as equivalents to the Firebug and Web Developer Firefox extensions. I haven’t had to do any HTML/CSS debugging for a while.
2. AdBlock Plus equivalent? I think there might be one out there, but I have to admit I’m not hunting since I’ve kind of fallen out of love with auto-updated ad blocking, it was blocking things I wanted to see sometimes. Opera does have a “content block” feature and I use it to block out particularly bad ads and ad-iframes (anything I see that is large, or overtly bright, or animated.)

I haven’t spent much time looking into equivalent Opera functionality as I have everything I need for day-to-day browsing “out of the box.” However, I just found “Opera equivalents to Firefox extensions” and “Opera equivalents to Firefox extensions II” which look like a pretty good starting point!

In conclusion: I’m converted to Opera. It has it’s downsides, but so does Firefox and my feeling at the moment is that Firefox is more annoying than Opera.