While this might not be news for everyone, reader Brad and I seemed to be having the same problem: Using Firefox 3, we would see the "unsupported browser" message pictured while trying to use me.com.
Obviously, Firefox 3 is a supported browser for MobileMe. A TUAW colleague gave it a try and found everything to work fine. Brad and I compared notes about what plugins we have in common, and found that Ubiquity was causing the problem.
It turned out to be related to the user-agent string: MobileMe uses the user-agent string to determine what browser you're using, and Ubiquity tacks on an extra bit at the end. For example, without Ubiquity installed, it shows Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:126.96.36.199) Gecko/2008120121 Firefox/3.0.5. With Ubiquity installed, it said Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:188.8.131.52) Gecko/2008120121 Firefox/3.0.5 Ubiquity/0.1.5. (Emphasis mine.)
Since MobileMe doesn't know what to do with the extra Ubiquity text at the end, it thinks you have an unsupported browser, and shows you that error.
The good news is that you can fix this, thanks to a simple technique from Tim Jarrett, and still use both Ubiquity and MobileMe. Read on to find out how.