One of my little habits is to have lots of tabs open. Browsing LJ produces about thirty per day, with interesting stuff / posts to respond to later. These add up :) and it's not unusual to have six or seven hundred open.
Firefox 2 had problems with this. I've got plenty of RAM, so FF2 taking just under 1G wasn't a big issue, but it could get slooow to open a new tab and, especially, to download something. CPU usage would go to 100% and I couldn't do anything with FF2 for several minutes. I more or less had to set the FF2 process to 'below normal' priority, or it would interfere with everything else. Restoring all of the tabs after a restart took a while too.
FF3, on the other hand, is still usable! It can download things and keep working! Reloading a couple of hundred tabs takes about two minutes, not over ten!
The add-on that makes this possible, Tab Mix Plus, isn't officially working with FF3, but you can get a development build from the developers here. Highly recommended.
Oh, I forgot to also update the AdBlock Plus add-on, which filters out lots of ads. My ghod, some sites are hideous without it - I'd forgot just how many ads they want to have. Even more recommended.