1. Download the latest official version of Firefox from Mozilla.com.
         * You should avoid unofficial download sites that bundle Firefox with other software such as the Google Toolbar. 
  2. If you wish to retain your current profile, you should make a backup copy of it. See Backing up your information for instructions.
  3. Uninstall Firefox and make sure you remove the Firefox application (program) folder and your Firefox personal data (user data and settings) - see Uninstalling Firefox for instructions.
  4. Reinstall Firefox - see Installing Firefox for instructions.
  5. When Firefox is started for the first time, the Import Wizard will appear and a new profile will be created.
  6. Verify that the issue no longer appears.
  7. If you made a backup of your old profile and wish to restore your old information, see Recovering important data from an old profile. For troubleshooting purposes, only copy the files corresponding to your bookmarks and saved passwords.
  8. Reinstall your extensions and themes one by one to determine if any specific one is causing the issue. 

Hi there, You could try performing a clean installation: [[Knowledge Base Article]] : Some Firefox issues can be resolved by downloading a new Firefox installer from Mozilla.com, then closing Firefox, deleting the Firefox application folder (program files) and reinstalling Firefox. If this does not help, or if you would rather completely remove Firefox and start over instead of completing all the troubleshooting steps, follow these steps: 1. Download the latest official version of Firefox from Mozilla.com. * You should avoid unofficial download sites that bundle Firefox with other software such as the Google Toolbar. 2. If you wish to retain your current profile, you should make a backup copy of it. See Backing up your information for instructions. 3. Uninstall Firefox and make sure you remove the Firefox application (program) folder and your Firefox personal data (user data and settings) - see Uninstalling Firefox for instructions. 4. Reinstall Firefox - see Installing Firefox for instructions. 5. When Firefox is started for the first time, the Import Wizard will appear and a new profile will be created. 6. Verify that the issue no longer appears. 7. If you made a backup of your old profile and wish to restore your old information, see Recovering important data from an old profile. For troubleshooting purposes, only copy the files corresponding to your bookmarks and saved passwords. 8. Reinstall your extensions and themes one by one to determine if any specific one is causing the issue. source: https://support.mozilla.com/en-US/kb/Basic+Troubleshooting?s=clean+installation&as=s#Clean_reinstall