I have found 2 bugs with Firefox 3.5 and it looks like they aren't going to fix them before release.
1. the url bar drop down text is unreadable. To fix install the addon stylish and then install this style: http://userstyles.org/styles/13324
2. fullscreen flash crashes the browser. This will happen if you installed the latest version of Flash 10 from Adobe. If you are using the flash that came pre-installed with Mint Linux it works fine. If you installed the latest from Adobe, click on the settings and uncheck hardware acceleration.