Browsers Circumvent Web Filtering

[#1]

We have been getting reports that people are able to get to blocked content using Firefox 4 and flavors of Chromium such as SRWare Iron. All IE versions, Safari, and Google branded Chrome " obey" the content filtering. Any ideas on how I can troubleshoot this other than having to monitor and write filters for all new user-agent strings that come along? We are running FortiOS Version 4.0 MR2

[#2]

Using application control to block gmail behaves the same way. Firefox is not blocked!