Firefox 4 is a memory hog compared to Firefox 3.6. Worse, it leaks memory like a sieve. After a bit of browsing, it is using 350 MB with just a single tab open. Firefox 3.6 uses half that much memory on the same machine.
Turns out, in Safe Mode it was perfectly well behaved, and after hitting the 'reset all preferences to defaults' button, Firefox not only takes less memory, but is snappier too.
I hate saying this but Firefox 4 is shaping to be a disaster of epic proportions. The Javascript benchmarks are all nice and good but dear Mozilla folk you people also need to write a garbage collector for your implementation of Javascript that actually collects garbage.