It sounds like maybe the update didn't download properly and your Firefox installation may be missing a file or have a damaged file. In this case, I'd recommend performing a Clean Reinstall.

Certain Firefox problems can be solved by 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: (Note: Check both folders.)
     • 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.

Hope this helps.

As an addition to the previous, I can't even OPEN firefox to find out what version it is.

I'm on a Mac using 10.15.5

Hi! I noticed that you marked my answer as the solution. Do you still require help?

Thanks Wesley... I've now downloaded the app and installed 3 times. Today it looked like it was working, but just now I tried opening a new tab and it refused to do anything. This is on chrome once again.

It's frustrating enough to make me go back to safari. Sorry to be such a pest, but...

A post was flagged as Solved Problem. If the problem is not solved, please remove that flag.
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Go to that message, and use the Undo link to its right.

Do you have security software installed on your Mac that could be causing this ?

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Thanks for all this help. I've refreshed firefox and I'm now trying out all the things I was doing when it failed.

So far so good...

I'm sorry to say this but while a page of my local newspaper was loading it simply stopped and the other 2 tabs failed to respond.

If there's no other fix can the older version be retrieved?

Use these links to create a new profile. Use this new profile as is. Is the problem still there?

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

I created a new profile and when I opened firefox I got this:

'The application “Firefox” is not open anymore.'

So now I've restarted my mac and opened firefox with my new 'as is' profile. The exact same thing happened, but faster this time. It wouldn't even open a new tab without crashing.

Well, thanks to everyone who tried to fix this problem.

Start your Computer in safe mode with network support. Then start Firefox.
Try Secure websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode
Starting Any Computer In Safe Mode; Free Online Encyclopedia