Marcus...
Did you recently update Firefox? I have used Firefox for years and have in the past experienced issues after an update. Try going back to the previous version you were running. That is what I did. The version of Firefox I am using now is quite old, Version 12. I experienced major issues when I updated it to Version 13 so I went back. I have had no issues since so the desire to go to a current version has long since waned. Especially now that they called each new version a Beta.
You may also want to search the Mozilla knowledge base. There may be information there that could be helpful.