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.