Assuming you have restarted Windows during that long period of time, I would suspect a locked file. Could you check the steps in this article to see whether they help?

"Firefox is already running but is not responding" error - How to fix

Hello,

Usually this can be fixed with a clean install. There have been important updates since that time, so make sure you download a fresh copy of Firefox from:

• https://www.mozilla.org/en-US/firefox/new/
• (or if you need a different localization) https://www.mozilla.org/en-US/firefox/all/

Then uninstall Mozilla Firefox from your Control Panel > Programs and Features/Add or Remove Programs. If you want to save your profile information, make sure that you do not delete personal files.

Delete the Firefox folder and all of its contents (your profile is located in a different location so will be unaffected by this):

• C:\Program Files (x86)\Mozilla Firefox\

Then reinstall Firefox using the installer you downloaded earlier. Does that fix the problem?

A clean install didn't do the trick. Thanks anyway.

Did you try the article jscher2000 posted?

If that article doesn't help, what happens if you run the profile manager for Firefox with the -no-remote parameter?

http://kb.mozillazine.org/Opening_a_new_instance_of_your_Mozilla_application_with_another_profile

Create a new profile as a test to check if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox