From MozillaZine Knowledge Base

If Firefox or Thunderbird starts in Safe Mode every time it is used, right click on the icon you use to launch the application, go to "Properties", and make sure the "Target" field does not have -safe-mode on the end. Using Firefox installed to the default directory as an example:

"C:\Program Files\Mozilla Firefox\firefox.exe" -safe-mode

If it does have -safe-mode on the end, remove the safe mode entry so it looks similar to this:

"C:\Program Files\Mozilla Firefox\firefox.exe"