We recently had issues with a lot of our Rspec tests suddenly failing.  We were getting a Selenium Error that there was a Modal dialog present.  After a lot of wasted time troubleshooting, it was determined that FF v. 13 was causing the issue.  It appears that this was an issue last fall (http://support.mozilla.org/en-US/questions/792131)  After following the recommendations from that forum article, we still had the modal dialog issue.

Our fix was to revert to FF v. 12 until they get things fixed.

Update: I also tested FF v. 14 and the same issue persists.