This is the result of some add-on or proxy messing with your User-Agent. You can see what your current User-Agent string looks like by visiting
this page. If you are lucky, that might provide some clue as to what's responsible for the problem.
Regardless, there are two places you should look to try to fix it:
1) Tools, Manage Add-Ons: examine and disable add-ons one at a time, starting with any you don't recognize.
2) Tools, Internet Options, Connections: examine connection-specific and LAN Settings for any proxies.
