Every so often, Firefox extensions go bad. Now they don’t start to live a life of crime – but they do tend to get a little buggy. The folks over at Mozillazine.org have a list of problematic Firefox extensions, and what you can do if you run into the same problem with those extensions.
This article lists known extensions that cause unwanted side effects in Firefox or in other extensions and plugins, along with recommended workarounds. For other issues related to specific Firefox plugins, see the linked article.