Apple's MobileMe recently launched rendering the .Mac services obsolete. Whilst the new MobileMe web apps work perfectly on Safari and on Mac and Windows, it does not work on Firefox on Linux… not yet, at least.
A workaround is pretty obvious – no, not installing Windows or buying a Mac, silly! You just need WINE to run Safari and/or Firefox. Here's a short how-to that might help you when installing Safari on WINE. Firefox on WINE is pretty straightforward and this, I can personally say, works!