I installed firefox-4.0 (currently 4.0b8pre) on Ubuntu Maverick RC earlier today and all was going fine and dandy until I had to open a Google Docs spreadsheet. I’d receive a message stating:

The bad news is that Google Docs has just encountered an error.
The good news is that you've helped us find a bug, which we are now looking into.

We apologize for any inconvenience this has caused you.
In the meantime, if you'd like updates on this and other issues, try visiting the Google Docs Help Forum.

Sorry, and thanks for your help!
- The Google Docs Team

I checked documents and they seemed to be fine. It turns out this is an issue with Google not recognising FF4’s user agent string. The easiest way to solve this issue is to type about:config into the address bar and set

general.useragent.compatMode.firefox

to true.