1. Tried changing some about:config options. Didn’t fix the beachball problem, but I guess it won’t hurt either.
2. Knowing that Adobe Flash is a memory hog, and particularly on the Mac, I installed the Firefox Add-on Flashblock. What it does is replace any flash element on a page with a little play icon. Click the icon to load the flash file. The only time you really want to see flash content is on a YouTube-like site, or maybe if it is part of a mandatory navigation menu. I recon this is what did the most impact for me, as I always have numerous tabs open, and some of them have flash.
3. Replaced the official Firefox build with one optimized for my Intel CPU. Minefield is available in builds for G4, G5 and Intel Macs respectively. The only difference from the official version, apart from the name and icon (for trademark reasons) is the way the code is compiled. Minefield is not going to have bugs that the official version doesn’t, or vice versa. Minefield will use and save to the same files as the official version, so you keep your bookmarks, tabs, history… the whole shebang.
I would definitely recommend trying out at least the 2. and 3. tip – the first one didn’t seem to make that much of a difference. Let me know what you think and what your experience is!