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.
The other day, I discovered a Firefox nightly using a gig and a half of memory after some light browsing; after some investigation it turned out that had a few hundred meg allocated immediately after startup (having loaded only about:mozilla) and it allocated a few hundred more when I opened and immediately closed the bookmarks and history menus.
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 beat the 350 MB by going to the Atlas Obscura website. Firefox managed to use up 550 MB with only a single tab open. This too after a clean start. Firefox just keeps on leaking memory when the Atlas Obscura website is open.
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.
