I've been using Firefox 3 since the first beta and it's a very good and quite stable browser. However, there's one "enhancement" that bugs me a lot. And that is the history+bookmarks combination that they call "Places".
Someone on a modern computer will probably not mind the performance impact and can see the uses, but if you are on a low spec laptop (yeah, I am) it's a whole different situation. To put it in more technical terms, places "can have a non-trivial impact on the browser's performance."
For me, Firefox with the default settings will have a lag from a fraction of a second to several seconds every time I edit the location. Clearly, that's unacceptable. There's no option to change places behavior through the menus, but you can do it from about:config and here's how:
browser.urlbar.search.chunkSizefrom the default 1000.
browser.urlbar.search.maxRichResultsfrom 12. 0 will give the best performance, but no rich results.
browser.urlbar.search.timeoutfrom 100. This is the wait between chunks.
My values of choice are 100, 3 and 300, respectively. You should test to find the best for your setup. That should take care of the location bar lag. Another reason for FireFox not responding is the background frecency calculations performed to keep places up to date.
places.frecency.updateIdleTime from 60000 (1 minute) you
can ensure that these background calculations only start when you are
not on your computer, instead of causing lack of response when reading
long blog posts.
These changes are enough for my computer. With these my Firefox is actually quite responsive until I start other large applications. Of course, there's little to do on that front.