Firefox can develop problems like any other application. Many problems are related to extensions and add-ons. Some relate to profiles becoming corrupted in some way. Some malware installs hidden extensions that cause popups or other problems.
Make sure that Firefox is closed and that Thunderbird is also closed if you use it.
Restart Firefox in Safe Mode for Windows users
Start > Programs > Mozilla > Firefox (Safe Mode)
If it isn't listed you can start it from Start > Run
When it opens you'll be presented with an option screen
Check the button “Make Changes and Restart” - This restarts Firefox in normal mode and applies the changes in any of the options selected above.
Warning: Don't continue to run in Safe Mode and start changing settings or adding extensions. That will make your changes the default Safe Mode and would make Safe Mode useless for repairs and fixing.
Other Operating Systems like Linux or Apple OS X are covered in the links