The error message being displayed to AdWords advertisers was:
Support for Firefox 3.5 is coming soon. Until then, please use an earlier version of Firefox when viewing the new interface.
Google fixed it a day later, and we have confirmed reports in that thread that the new AdWords interface is fully operational in Firefox 3.5. Oh, and by the way, it is also now working in Safari 4.0.
Forum discussion at Google AdWords Help .