Did you do a 'clean' removal of Firefox? This means you open the Programs Folder and rename or delete all Mozilla or Firefox folders.

Do not change the Mozilla Thunderbird folder if there is one.

I deleted the Programs folder that said Mozilla Firefox and uninstalled the program (for the third time). No luck. Downloaded and reinstalled Firefox and still get the same error message.

I don't understand what the computer is looking for that it suddenly cannot find any more

What are you installing, the 32 or 64 bit? Try loading the other.

Try to start Firefox in Safe Mode to see if that works.

• Windows: hold down the Shift key while starting Firefox
• Mac: hold down the Options key while starting Firefox

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

You can create a new profile to test if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

FredMcD said

What are you installing, the 32 or 64 bit? Try loading the other.

Thanks FredMcD, but no luck. Running the 32-bit installer results in the 64-bit being installed, Must have auto-detect for the appropriate system.

cor-el said

Try to start Firefox in Safe Mode to see if that works... Thanks, Cor-el, but no luck. This seems to be a strange one. I deleted remaining Firefox folders as FredMcD suggested, and even ran CCleaner. Then reinstalled and started as you suggest using the shift key. A box asks if I want to start in safe mode, I click yes, and I get the same error message, but also the "well this is embarassing" restore tabs screen. And, in the task bar, there shows up a second, blank Firefox tab (are they called tabs down there?). Both have the FF logo, but one tab has a page name that says "restore session - Mozilla Firefox" and the other tab is blank. The blank tab is not related to the error message box. When I click on it, nothing happens

It doesn't even start in safe mode. I don't see any other Mozilla folders (besides Thunderbird, which is installed on a separate drive). Is there some other place where something might be hiding that has to be deleted?

UPDATE - tried installing Firefox 12 and here's what happened

I have three drives in this computer. C, K (all the current stuff, including most programs) and D (archives). In the programs file on the K drive I found the installer for Firefox 12. I installed that, selecting one of the two profiles that showed up in profile manager. 12 then updated automatically to version 43, which updated automatically to version 47. These appeared to work. It reloaded the bookmarks and everything.

THEN, version 47 updated automatically to 56 and immediately failed during startup - and I got the same error message as before.

Has this happened to anyone else?

And finally, I went through the old versions of FF until I got to version 47. Then told it to stop looking for updates.

Looks like something is about version 56 is not liked by my computer. But how can this be? How can an older version run and the latest not?

If I have to keep an outdated version in order to have Firefox, then I will, because even in outdated form it's better than Chrome and the best browser out there.

I will move this to a new question, as it is a little different from the other problems where people couldn't run version 56

You have several choices. Keep your current without updating.

https://www.waterfoxproject.org/ Waterfox - Official Site

Firefox; Extended Support Release {web link} ESR Notes System Requirements