We have seen this issue a lot of late. I don't know if this is your problem:

You can check for issues caused by a corrupted or incomplete Visual C++ installation that is missing runtime components (Redistributable Packages) required by Firefox, especially VCRUNTIME140_1.dll.

Install both 32-bit and 64-bit "Visual Studio 2015, 2017 and 2019" downloads, (x86: vc_redist.x86.exe and x64: vc_redist.x64.exe) from https://support.microsoft.com/help/2977003/the-latest-supported-visual-c-downloads

You may not need the latter (32-bit version) if you use 64-bit Firefox. You may have to reboot the computer. ++++++++++++++++++++++++++++++++++++++++++++++ In another thread, a user reported that a recent update of Malwarebytes Anti-Exploit (version 415) may be causing symptoms similar to the VC Runtime problem. If you have MBAE, it should have a setting to exempt Firefox. You could test whether that makes any difference.

Let’s do a full clean re-install;

Note: Firefox comes in three or more folders on all computers. They are;

Maintenance: (Programs Folder) <Windows Only> Firefox itself: (Programs Folder) And two folders in the profile of each user on the computer for each Firefox profile for that user.

If you remove the Firefox folder, the user profiles would not be affected.

Download Firefox For All languages And Systems {web link}

Firefox ESR; Extended Support Release {web link}

Beta, Developer, Nightly versions https://www.mozilla.org/en-US/firefox/channel/desktop/

Install Older Version Of Firefox {web link} Be sure to read everything here.

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.

How to Start all Computers in Safe Mode; {web link} Free Online Encyclopedia

Hi FredMcD, thank you for the swift assistance Unfortunately reinstalling Visual C++ has proven ineffective I am not currently utilizing malwarebytes And the secondary post about a clean installation was one which I found in several other locations and have followed unsuccessfully

Update: have attempted to utilize a newly created profile on a different PC with functioning firefox However the same "An unexpected error has prevented your changes from being saved." message prevents me from starting up firefox

Update 2: After uninstalling Firefox, booting up in safe-mode and installing it during safe-mode Firefox works during safe mode

However

In normal mode I get the following message each time I attempt to start up firefox I get the follow error message

"Firefox is already running, but is not responding. The old Firefox process must be closed to open a new window."

No firefox process is opened when this message is received

Although you are likely right with that Firefox was not running, please try Taskkill /IM firefox.exe /F to kill all Firefox processes

Although you are likely right with that Firefox was not running, please try Taskkill /IM firefox.exe /F to kill all Firefox processes

Attempted this and the result was that no active firefox porcesses were available to be closed, unfortunately since it would have been an easy fix.

I would not normally recommend this, but could you try deleting the file located at %appdata%\.mozilla\firefox\{profile name, likely default-release}\lock

I have attempted that unfortunately it did not work I also had a different computer problem pop up in the meanwhile which forced a restart, since then attempting to start up firefox does nothing, not even produce an error message

Could you try backing up and then deleting the default profile to see if that is the issue?

If you have done so, maybe back up the entire .mozilla folder and start completely fresh

I have tried to get rid of the entire .mozilla folder in %appdata% and starting entirely fresh, however in this state the original issue existed

If attempting to delete all mozilla presence in %appdata%, then reinstalling, the follow happens

Firefox opens, detects no profiles, starts up profile manager Prompts me to create a new profile Profile Creation Wizard activates for a split second then closes down automatically

After taking a clean new profile made using firefox on a laptop where it functions, and transferring it entirely I get the following issue once more when starting up firefox

"An unexpected error has prevented your changes from being saved."

I have attempted to install other Mozilla products and those also have account issues. Further, while I cannot find an active firefox window or application anywhere, when trying to import bookmarks from firefox to chrome (as a hopefully very temporary solution) chrome indicates it cannot import bookmarks since firefox is still active

I called for more help.

I called for more help.

Thank you for the assistance so far, I have tried to roll back my pc to as far a date away as I dare (about a month) without any luck, and given the issue only occurred today (found at around 9am GMT) I don't think this course will benefit more.

Tried selectively turning off various malware and other protections since I managed to get it to run during safe-mode but unfortunately this has not resulted in any progress.

I have also tried to scan my pc for any mention of firefox or mozilla after uninstalling and removing those files to try clean again, but I will admit I was not likely thorough in this.