If you are looking for an older version of Firefox, you can get all the versions here
»ftp.mozilla.org/pub/mozilla.org/ ··· eleases/
However, my suggestion would be to start with a fresh install of Firefox using the latest version (18.0), and use a new profile, and see how that goes.
Depending on how long you have used it, and how many extensions you may have, there is a chance that one (or more) may no longer be supported. I've also had cases where a profile just has issues, and odd things start happening. Going with a new profile most times will fix the issues.
I've been using Thunderbird and Firefox for at least 4 years now. However, every so often when a new version comes out, instead of doing an upgrade, I'll do a fresh install to a new profile. Of course, I may take things a bit extreme, where I'll only save my bookmarks, and then dump everything else. Since I don't have any extensions, and only 3 plug-ins (Java, Flash, and Silverlight), its easy for me to set everything up again.
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