You may want to create a new profile to see if there is a problem with your profile or some extensions you can do this to test for problems.
Click on the Start Button and if you have it on the right side of the start menu click on Run
if it is not there type "run
" without quote marks in the search box and choose Run
In the Run box type "firefox.exe -ProfileManager
" without quote marks and click OK
Click on Create Profile you can also uncheck don't ask at startup now.
Follow the Wizard and click Next.
Enter new profile name and click Finish.
Make sure that you have unchecked don't ask at startup and select your new profile and click on Start Firefox. This profile manager box will open each time you start Firefox until you check the Don't ask at startup box again.
After you have tested your new profile you can check the don't ask at startup box again and you will not get the profile manager starting each time you start Firefox.