It could be a corrupt profile.

1. You can start Firefox in Safe Mode by holding the Shift key when starting then application. See if starting with add-ons and some settings disabled allows FF to start and run.

2. You can start Firefox profile manager by typing firefox.exe -P in the Windows search box in the start menu, or in a cmd window.

Create a new profile and start FF using the new profile, and if it starts and runs without issue, you can transfer your bookmarks and logins to the new profile.

Edit: These options may also be present as shortcuts in the Start Menu's Firefox folder.

You may also be able to fix issues if you are able to open your existing profile in safe mode by refreshing Firefox. https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings

I have tried both of these. Firefox will not start in Safe Mode and I cannot open Profile Manager. I even tried deleting all the profiles and the profile.ini file and it still won't open.

Not sure whether this could be relevant:

There appears to be a known issue among users that have previously had Comodo security software on their system. The conflicting file is an older version of:

C:\Windows\system32\iseGuard64.dll

I don't use Comodo security software. I did find a post on another forum that was having the exact problem and also couldn't enter Firefox in Safe Mode. Someone suggested a problem with a Logitech camera driver being outdated. I uninstalled the software and the problem was solved! It was strange that I used Firefox with no issues on Wednesday, but when I turned my computer on Thursday it suddenly didn't work and there were no updates installed. Thank you for your suggestions!

Creating a new Firefox profile and starting with that and deleting the default one appears to have solved the problem. FF is not crashing anymore. Thanks