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

• Disable all add-ons - check
• Reset toolbars and controls - check
• Reset bookmarks to Firefox defaults - Leave unchecked unless you have a backup of your bookmarks.
• Reset all user preferences to Firefox defaults - check
• Restore default search engines - check

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

Firefox Safe Mode - Further Information
Firefox Safe Mode - Slightly Easier to Understand