I use Mozilla Firefox because of the great tab control, automation, nice bookmarks toolbar, no text aliasing, other add-ons, and general speediness. But it slows down a lot as I leave one window open for days (using Windows’ hibernation function at night); it’ll even lock up and make everything else slow as molasses, as it steals all the CPU cycles and RAM. I did re-install recently, switching from the portable version to the regular Windows installer, but to no avail. It surely doesn’t help that I like to keep 20 tabs open.
Firefox was originally supposed to be simple and fast, replacing the slow Mozilla suite, but with spell-checking, RSS support, history caching, etc. in the core instead of being relegated to add-ons, it’s becoming increasingly heavy-weight. Inefficiencies in the code don’t help either, though changes will mess up the plugins people love, I hear.
Incidentally, there is the same problem of slowness with WordPress; I and many other users have to use caching to stay running during traffic spikes. Though most database-driven websites use caching, it’s particularly necessary due to WordPress’ inefficiencies. And they won’t be going away or else plugins will break. At least we get lots of cool features out of it.