Firefox3 userAgentButton is null

While working on phpFox2 today I noticed the following JavaScript error on each page:

I first thought it was a compatibility issue with jQuery and some new JavaScript code I added, however looking over the logs I didn't add any new JS code so that did not seem to be the problem.

Many problems I have faced in the past when dealing with Firefox was because of plug-ins so the next thing I checked was if I had any plug-ins related to modifying user agents, which I did. I installed the add-on User Agent Switcher a few weeks ago. I disabled it and restarted Firefox and everything was in order. I didn't bother to look for an update for the add-on as I haven't used it since the first time I tried it out, however I am sure there is another fix if anyone plans on using the add-on.