press CMD and TAB and kill Firefox, or was it CMD and ESC

I tried that it didn't work. Thanks anyway.

Do you still have a default Firefox profile folder?

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

Did you move the Firefox application to the trash?

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

See "Basic Troubleshooting: Make a new profile":

• https://support.mozilla.org/kb/Basic+Troubleshooting#w_8-make-a-new-profile

If the new profile works then you can transfer some files from the old profile to that new profile, but be careful not to copy corrupted files.

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

I tried it didn't work. Thanks anyway

I could not open Firefox 13.0.1 when I updated to Mac OS X 10.7.4 I receive the following prompt:

A copy of Firefox is already open. Only one copy of Firefox can be open at this time.

I created a new OS user: System Preferences> Users & Groups> click on the "+" symbol, bottom left to create new user & save profile.

Log out of current profile and log in to new OS user profile.

Open Firefox in new user to see if the application opens.

In my case the application opened. I then logged out of the new user profile and logged back into my old profile & followed the directions in the following weblink: The following link resolved the issue: http://kb.mozillazine.org/Profile_Manager#Accessing_the_Profile_Manager Scroll down to: Accessing the Profile Manager > Mac OS X Close the application completely and make sure that it is not running in the background. Assuming the program is installed in the "Applications" folder, launch Terminal ("Applications -> Utilities -> Terminal") and enter the command starting with / after the prompt in Terminal: (Firefox) /Applications/Firefox.app/Contents/MacOS/firefox-bin -profilemanager I then deleted the old profile and created a new profile. I was then able to open Firefox.