http://kb.mozillazine.org/Firefox.exe_always_open

https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding

https://www.basezap.com/cant-open-firefox-because-a-copy-of-firefox-is-already-open-mac/

https://apple.stackexchange.com/questions/196771/cant-open-firefox-because-a-copy-of-firefox-is-already-open

Those are all interesting links, and pretty much cover all of the things that I've already tried (except for the first link which was Windoze related). As I mentioned in the details for my question, i tried deleting the process (which doesn't exist) and deleting .parentlock (which isn't the issue).

So, anybody got any ideas?

Is there (for example) a way to COMPLETELY uninstall Firefox? Maybe that would be an option?

Let’s do a full clean re-install;

Download Firefox For All languages And Systems {web link}

Save the file. Then Close Firefox.

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86) Note: Check Both Folders

Mac: Open the "Applications" folder. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

Linux: Check your user manual. If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it. See Install Firefox on Linux; https://support.mozilla.org/en-US/kb/install-firefox-linux

If you downloaded and installed the binary package from the Firefox download page, simply remove the folder Firefox in your home directory. http://www.mozilla.org/firefox#desktop ++++++++++++++++++++++++++++ Look for, and remove any Mozilla or Firefox program folders. Do not remove the Mozilla Thunderbird folder if there is one.

Do Not remove any profile folders.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. +++++++++++++++++++++++++++ If there is a problem, start your Computer in safe mode and try again.

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode This shows how to Start all Computers in Safe Mode; Free Online Encyclopedia

I finally managed to solve this problem!

I tried pretty much everything that I could find documented on the web, and even did the full clean reinstall as suggested, but that didn't work either. To be honest, I didn't expect it to because the instructions didn't include any information on uninstalling the current version therefore I guessed that there'd still be stuff lying around. Sure enough, when I'd done reinstalling a new version of Firefox, voila, same problem again.

:-(

So, I did yet more research.

Almost every reference was focused on deleting the .parentlock file or killing the process from Force Quit. I was getting seriously frustrated!

But, eventually, I came across an article on Firefox profiles and how it was possible to access them without opening Firefox. I thought to myself, can't hurt to take a look.

The command prompt I found (and subsequently typed into Terminal) was:

/Applications/Firefox.app/Contents/MacOS/firefox-bin -P

As if by magic, this opened a window in which TWO Firefox identifies were listed. One was just titled 'default' while the other was 'default-9999999999999'. I should point out, it was 13 different digits to those that I listed. Hmm! Maybe this was the problem.

So, I did eeny-meeny-miney-moe, selected the first profile, and deleted it. Pure guesswork but, hey, FF wasn't working anyway so nothing to lose.

I then re-started FF ... and it worked!

Hooray.

I don't know why it worked, but it did. Maybe somebody with knowledge of both Macs and Firefox will read this and can explain what was happening and how I (accidentally) fixed it.

All-in-all, I'm pretty tired of Firefox. It's a slow and unstable browser, or at least it is for me. Even worse, the support availability for it is ... well ... non-existent. I genuinely hope that my solution above helps somebody else at some point. Unfortunately, I have no idea why it worked, I only know that it did.

That was very good work. Well done. Please flag your last post as Solved Problem so others will know.

I would think that this worked because the profile that Firefox tried to use by default was corrupted. By removing this corrupted profile and selecting a not corrupted profile you got it to work. I don't know what as wrong with the other profile that it trowed this error message, but I assume that selecting the other profile in the Profile Manager without deleting the other profile would have worked as well.

sylvester-13 thank you for the solution I created an account to add that firefox asked to refresh firefox and update its appearance so i clicked this option and it took quite a while to do it so i canceled the process and then i ended up with this error and you solution worked perfectly. I believe the profile with the numbers is unique and should not be deleted but the profile with no numbers is the profile to be deleted it as i was able to get all my tabs and bookmarks and all my information back.

total insanity here 17,000 people complaining and firefox doesn't fix this error message. You close a web page and try to open another one and get this message" firefox is already running and needs to close. " So tried of reading different peoples responds that they " DON'T KNOW WHY IT WORKED" . I don't want to spend 9 hours reloading and resetting firefox just to make it open a web page. Thats BS. FIREFOX needs to get off its little rear end and really start to set up a real time real person help section. With real people solving these problems, not just some volunteer group of people that don't know why it worked ????? FIREFOX ?????????? SPEND SOME MONEY AND FIX THIS ERROR MESSAGE.

The people who answer questions here, for the most part, are other Firefox users volunteering their time (like me), not Mozilla employees or Firefox developers.

If you want to leave feedback for Firefox developers, you can go to the Firefox Help menu and select Submit Feedback... or use this link. Your feedback gets collected by a team of people who read it and gather data about the most common issues.