Maybe this will help.
"Recreate Firefox Profile
If none of the methods above work, it might indicate that there is some problem with the Firefox user profile. To fix the problem, you will have to create a new Firefox user profile. The user profile is used by the Firefox browser to save your personal settings such as passwords, bookmarks, and extensions.
Close all windows of Firefox browser open on the desktop.
Next, open the Start menu and select Run.
In the Open field, type firefox.exe -ProfileManager and press the OK button.
Select the existing corrupt profile and select the Delete Profile button to remove it.
Next, select the Create Profile button, and follow the screen instructions to create the new profile according to your requirements.
After creating the new profile, you may proceed to uninstall Yahoo toolbar using one of the methods discussed above.
You may want to uninstall Yahoo toolbar from Mozilla Firefox if you are encountering frequent problems while working with it. You can remove the toolbar either by using the option available on the Pencil menu of the toolbar or by using the Firefox Extension manager. If you are unable to remove the toolbar using one of these methods, you may have to create a new Firefox user profile using the ProfileManger utility. After the uninstallation, you may also use a registry cleaner utility to scan the system registry and remove any obsolete entries or vacant spaces left behind by the toolbar. It is also recommended to keep your antivirus and anti-spyware software updated with latest virus definitions. This will enable you to ensure that no malicious code or files creep on to your PC and cause damage to your precious system."
Methods to Uninstall Yahoo Toolbar From Firefox at Instant Registry Fixes