Thanks to Grail Knight

How do I reinstall Firefox v3?

Firefox Clean Install Instructions for Windows OS.
Just removing the Firefox folder itself and installing a new build will not fix any sever issues you are having.
Follow the steps below to reinstall Firefox correctly.
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1. Download Firefox v3 (or current version)

2. CREATE A BACKUP OF YOUR PROFILE.
You can simply copy everything to another folder on your hard drive that will give Firefox the ability to retrieve your prior settings, bookmarks (places.sqlite), passwords (signons, key3), permissions.sqlite etc.
If you have customized your search plugins,back them up also. (They are in the installation directory, not in the profile).
(Newly added search plugins will be stored in the profile in Firefox v3 (or current version); you may wish to move the entire search plugins folder from the installation directory to your profile folder).

More information on the Profile Folder is located here: Fx Profile Folder.

3. Uninstall any previous version(s) of Firefox. In Windows use the Add/Remove Programs in Control Panel.
After uninstalling Firefox, Windows users should go back and delete the Firefox installation directory, typically:
C:\Program Files\Mozilla Firefox.

Note* If using a .zip version of Firefox proceed to the location you installed the .zip build and delete the entire Firefox folder.

4. Delete the Firefox folder in the Firefox "profiles" location. (Do not simply delete the "profiles" subfolder. Make sure to delete the Firefox folder containing the "profiles" folder and related files, including "profiles.ini).
Note* If using a .zip build proceed to the location that you have placed your Profile Folder and delete the Profile folder.

5. Delete the Mozilla Folders found here (One Mozilla folder in each location):

C:\Documents and Settings\YOURNAME\Application Data\Mozilla
&
C:\Documents and Settings\YOURNAME\Local Settings\Application Data\Mozilla

6. Install Firefox v3 (or current version).
You can reboot your PC so you have a fresh start and do not install any extensions or themes until you have v3 (or current version) successfully installed.

Move to Step 7 if reinstall of Firefox is successful.

7. BEFORE you open Firefox, copy key settings back from your prior profile(s). From your Backup profile folder (created in step #2). Copy over places.sqlite, cookies.txt, hostperm.1, key3.db, signon.txt, permissions.sqlite, formhistory.dat, userChrome.css, userContent.css, and user.js from your backup to your newly created profile.

Note: userChrome.css & userContent.css are located in your Profile in the Chrome folder and only need copied into your new Profile if you have customized them previously.

8. Start Firefox and make sure it is functioning correctly. Now reinstall any extensions and themes you may use.

Notes: Parts of this instruction list from Mozilla.