Normally, when installing Firefox, any older version is overwritten.

Sadly that is not the case and it appears that the old programme is interfering with any new Firefox installation.

You could try following these steps that were written by Mozilla to remove browser data and everything related to it.

Thanks Josh, that doesn't work. I've tried helper and the normal Win10 uninstall procedure.

What I have noticed is that the Firefox Uninstall Wizard does not open and the whole process stalls there.

Hmm... Can you try the steps in Windows "Safe mode with networking"? Or even try to install a new version of Firefox there?

I did install a new version of Firefox but it appeared to clash with the old version.

Can you try removing both of them in "safe mode with networking"?

Not sure how to do that.

UPDATE TIME! Thanks to Happy for providing instant link: https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

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)

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 ++++++++++++++++++++++++++++ Do Not remove the profiles folder.

Look for, and rename any Mozilla or Firefox program folders by adding .old to them. Do not rename the Mozilla Thunderbird folder if there is one.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. If all goes well, remove the OLD folders when you are done. +++++++++++++++++++++++++++ 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

Note: V55 has been giving users problems. Reinstall v54.

https://support.mozilla.org/en-US/kb/install-older-version-of-firefox?cache=no Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues.

You should also check out Firefox; Extended Support Release https://www.mozilla.org/en-US/firefox/organizations/ https://www.mozilla.org/en-US/firefox/organizations/notes/ https://www.mozilla.org/en-US/firefox/organizations/system-requirements/

Sorted!

Big thanks to you all for helping ............the power of the Community!!

David

Glad we could help you out :)