Have similar problem but after using Zone Alarm Internet lock. Try this. Close all open Firefox windows if any are open. Open Windows Task Manager. Click on Processes tab. Select firefox.exe (it will be there if it is running even with no windows open) then click on End Process. This should clear the running Firefox and allow a new one to open when you start again from a shortcut etc. without shutting down and rebooting your computer. A tiresome work around but it works for me.

I followed the step and firefox.exe will not respond when I click "End Process". So I also tried "End Process Tree" and firefox.exe did not terminate the process.

I was trouble shooting the error, "Firefox will not open" and the problem was a New Profile needed to be created. So I performed the following steps on Windows XP:

1. To open the Profile Manager 2. Click on Start 3. Selected Run 4. Typed in - firefox.exe -ProfileManager 5. A window will appear "Firefox" Choose User Name

NOTE: If Profile Manager does not display, Firefox could still be running. You have 2 options to close the program:

    a.  Restart the computer  
                      OR     
    b.  Open window Task Manager by selecting Ctrl, Alt, Delete
         i.  Click on the Process tab
         ii.  Locate  "firefox.exe"        
         iii.  Than click on End Process

6. To create a new Profile, select Create Profile 7. Than click Next 8. Enter the Profile name: Example - Your Name

NOTE: This information can not be seen on the Internet

9. If you want to choose were you want to store the new Profile,

      a.  Select "Choose Folder" 
      b.  Select File Destination
      c.  Create a new folder Titled "Your Name" Profile

10. Select Finish to create New Profile

Now launch Firefox. I hope this corrects your problem.