Although Firefox is considered very safe and stable, it sometimes hangs. In the past, when it hung, my solution was to remove the whole ~/.mozilla/firefox directory. It worked every time (reinstalling Firefox doesn't help). Of course, I would always backup my bookmarks.html and other files under the chrome sub-directory first. Then I had to reinstall the add-ons when I got a working firefox. It turned out I shouldn't have taken such a drastic action. According to this firefox documentation, sessionstore.js files are often the culprit. Removing them usually works for me.