Hi, hold down Shift when you start Firefox to start in Safe Mode. More - Diagnose Firefox issues using Troubleshoot Mode. If it works in Safe Mode, disable any theme or add-ons you have - Add-ons Manager (Ctrl+Shift+A) > Appearance/Extensions, then if it's OK, re-enable them one by one until you find the culprit.

If FF still won't start, try performing a Clean reinstall. This means you remove your Firefox program files and then reinstall Firefox. This process does not remove your Firefox profile data (such as bookmarks and passwords), since that information is stored in a different location.

To do a clean reinstall of Firefox, please follow these steps: Note: You might want to print these steps or view them in another browser.

1. Download the latest Desktop version of Firefox from mozilla.org (or choose the download for your operating system and language from this page) and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (or open the Firefox menu and click the close button ).
3. Delete the Firefox installation folder, which is located in one of these locations, by default:
   • Windows:
     • C:\Program Files\Mozilla Firefox
     • C:\Program Files (x86)\Mozilla Firefox
   • Mac: Delete Firefox from the Applications folder.
   • Linux: If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory.
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

More information about reinstalling Firefox can be found here.

WARNING: Do not use a third party uninstaller as part of this process. Doing so could permanently delete your Firefox profile data, including but not limited to, extensions, cache, cookies, bookmarks, personal settings and saved passwords. These cannot be easily recovered unless they have been backed up to an external device! See Back up and restore information in Firefox profiles.

Please report back to say if this helped you!

Thank you.

The above recipees did not work. Nothing ever opens - neither in Safe Mode nor after a clean reinstall.

Please copy/paste the bp- files after reading since you mention it crashes. https://support.mozilla.org/en-US/kb/firefox-crashes-asking-support

So this does not work ??? Please uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved.

Then install Firefox 56.0.2 from a Full Version Installer https://www.mozilla.org/firefox/all/

I have Norton Toolbar installed. Not a issue.

Please let us know if this solved your issue or if need further assistance.

I had done this all before. Nonetheless I went through the steps you suggested. Same result - Firefox will not run. As always there is about 3-4 seconds of activity (circling icon) and then nothing. This happens when you double-click in desktop icon or go to the directory C:\Program Files\Mozilla Firefox\firefox.exe and run that. I am just stumped.

I am a little puzzled why the properties of firefox.exe as shown under the security tab includes two extra Group or user names besides the expected SYSTEM Administrators Users

namely ALL APPLICATION PACKAGES ALL RESTRICTED APPLICATION PACKAGES

I don't see these for any other application. Clue?

If you are not getting any kind of crash dialog, either from Windows 10 or from Firefox (Mozilla Crash Reporter), Windows may be "killing" Firefox early in the startup for some reason. Perhaps...

A Recent Issue for 64-bit Windows Users with 64-bit Firefox

One of the main effects of the Firefox 56.0.1 update was to switch 64-bit Windows users from the traditional 32-bit version of Firefox to the 64-bit version. We've received a number of puzzling reports of Firefox either not starting (probably due to incompatibilities with 64-bit drivers that Firefox expects/requires) or not loading any pages after this migration.

One user discovered that Windows was silently killing Firefox at startup and recording it in the Windows event logs (you can view event logs using the Event Viewer). Based on one of the reports in the log, we were able to trace that to an out-of-date Logitech camera driver, of all things. Four other users reported that uninstalling the Logitech camera resolved their problem as well.

• https://support.mozilla.org/questions/1179822#answer-1019375
• https://support.mozilla.org/questions/1181740#answer-1021470
• https://support.mozilla.org/questions/1181405#answer-1022798
• https://support.mozilla.org/questions/1182572#answer-1023413
• https://support.mozilla.org/questions/1182847#answer-1024290

Do you want some assistance in looking at event logs, or do you have such a camera?

If the investigation doesn't yield any solutions, it is worth trying a return to 32-bit Firefox.

While normally this kind of change is seamless -- the installer detects your existing data and preserves everything -- you might want to make a backup "just in case."

• Back up and restore information in Firefox profiles
• How to opt out of 32-bit Firefox migration to 64-bit

Note: If you decide to try 64-bit Firefox on the same system at some point in the future, see: How to switch from 32-bit to 64-bit Firefox.

Yes, the problem was I do have a 64 bit system and a Logitech Camera! I installed the 32 bit version from the right area (some places say it's 32 bit but it was not!). Download the full version and don't use the standard install. This puts it in the x86 area. Everything now works. Thanks for your tip!

When you eventually update the camera software, then you will be able to take advantage of 64-bit Firefox.