It's been happening with Google too. I just dismiss the message and continue. I'm using FF 7.01.
Ya, I'm about ready to can FF myself after many years of devotion. These latest releases suck up system resources something fierce! I've been slowly getting comfortable with Chrome, not ready yet to make the change. I need to use the browser that the majority of our visitors use which at the moment is Firefox, IE and then Chrome way down on the list.
Also, I'm finding many times where stuff works in IE and Chrome but doesn't in Firefox. It drives me Dev nuts!
Well I used to be able to access adcenter from my mobile phone (android). Although i got the 'browser not compatible' window I could close that and continue to login and I would enjoy fairly good functionality but now I can't access it at all from my mobile. I even downloaded opera mini but still no good. Way to go adcenter! is there any way to access it from a mobile phone which surely isn't to much to ask these days?