browswer is not officailly supported by this game

I have never seen that message by using Firefox, Chrome and Chromium. What browser did you use?

Anyway, the thing is Tiberium Alliances is a web browser game that heavily relies on the browser's support for new web technologies. If your browser doesn't support some of the features the game needs, it makes sense you get that error. Also, make sure your browser is up to date: http://browsehappy.com/