One thing to keep in mind with FireFox and most other programs,
is that they run automatic updates in the background.
If you've had a version of the browser for some time it has had patch after patch after patch after patch after patch...
you get the drift. So the browser starts running slow or maybe a little glitchy.
Best solution is to do a fresh install.
But what about all your settings? Passwords? Bookmarks? How do you save all those?
There is an easy way to do it.
A utility named MozBackup will save everything for you.
Then you delete the old patchy version.
Do a fresh install of the latest version, then run MozBackup to restore your customization.
You can get MozBackup here: http://download.cnet.com/MozBackup/3000-2242_4-10260383.html