I downloaded the latest version of FireFox for Windows (3.5.5 at this time) and installed it on the latest version of Crossover (8.0.0) for Linux on Ubuntu 9.10. I am now using this browser to access Quickbooks Online without any problems at all. (I'm also using it to make this forum post.) QuickBooks Online requires a Windows browser -- I've tried using the Linux browser and spoofing the User Agent but I ran into issues on some pages. I've run into no problems using the Windows FireFox with Crossover.
Very nice!