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 ++++++++++++++++++++++++++++ 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

Just reinstalling Firefox solved the problem on the original account. However, the other account stills does not work at all (it does not appear to start even for profiles). I reinstalled to no avail. I have run ccleaner and fixed all the notified 'errors' and reinstalled. Finally I have tried installing in Safe Mode (including Networking).

see this answer:

• https://support.mozilla.org/en-US/questions/1182916#answer-1024501

The other account is not an administrator and, when I used an account which did (in their ID), unsurprisingly it worked fine.

I gave the other account administrator and tried again. It appeared to do SOMETHING but still doesn't work (even as administrator).

I have so far not found any error log entries.

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

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/how-run-firefox-when-profile-missing-inaccessible

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

Type about:profiles<enter> in the address bar.

I think that I have worked out what is going on.

I was upgrading from 32bit to 64bit with this update. This worked fine on the account with administrator but not with the one that was not administrator.

The issue seems to be that this update can only be done with administrator privaledges. When run as a non administrator, an admin account must be entered, this means that at least part of the update is not made on the requesting account.

By giving the other account administrator and then running the update again everything seems to work.

I guess that the update process should be changed to allow for this.

Thanks for the help

That was very good work. Well done.

Reinstalled firefox, opened thunderbird, yah it worked but only for a fraction of a second. Thunderbird was there but quickly replaced by firefox browser

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