First (this isn't an official Mozilla opinion, just personal experience), I find Webroot to be rather a poor quality anti-virus, with poor detection rates, and lots of problems when it does run. My opinion is to use Microsoft Security Essentials for anti-virus. It is free, light and quite good. http://windows.microsoft.com/en-US/windows/products/security-essentials. If you decide to go with it, uninstall webroot first.

Now, if you decide to keep webroot, you can try fully uninstalling it, then reinstalling it. Also, if you go to "About:crashes" in your firefox address bar, do you have any crash ids (they start with bp-).

Thanks for getting back to me. I did read, upon further research, that this is a known issue - some type of bug or conflict that exists between Firefox and Webroot - and that a fix will be provided in a future release of Webroot. So I guess there is no need to upgrade at this point and to continue using the older version. Others that have been recommended to me are AVG and Avast.