The only way you could have obtained that version of Firefox is by installing a ppa manually or through another application like Ubuntu Tweak, because you are using 10.10 and Firefox 8+ is only available to Natty and Oneiric users. The Ubuntu developers didn't upgrade Firefox without warning you, but that will happen eventually, when Mozilla stops providing support for Firefox 3.6.x, probably with the release of Firefox 10.
So all you need is to open Software Sources, click the Other Sofware tab, locate any ppa related to Firefox or Mozilla, disable them, then re-install Firefox.
However, if your problem is add-on compatibility then you could get compatible versions or force compatibility.Code:sudo apt-get install --reinstall firefox
You can get a Compatible version of CookieSafe (3.1a9) at https://addons.mozilla.org/en-US/fir...ible/versions/
Bookmark This Page Plus is a dead extension that hasn't been updated in more than 2 years. Forget about it. Use Add Bookmark Here ².
Autotrans is another dead extension that hasn't been updated in two years.Use Add to Transmission instead.
I strongly advise you to get rid of those old extensions and use the ones I recommended, because they are compatible and currently maintained. Sooner or later, when Mozilla stops providing support for Firefox 3.6.x, you will have to upgrade to Firefox 9+ and get rid of them. I also strongly recommend upgrading to Firefox 9 now, since it is a lot faster than 3.6 and has many features not available to your current browser. Use the firefox-stable ppa for that.
On Firefox 10, add-ons are now considered compatible by default. However, those old add-ons would be flagged as incompatible anyway.