OK, not finding anything new on that error, all seem to be archived.

Ok since was a Win 7 then a Win 10 lets check for Windows errors first.: https://support.microsoft.com/en-us/help/929833/use-the-system-file-checker-tool-to-repair-missing-or-corrupted-system

Now you can pick here since your used to uninstall and re-install you can download the full Firefox installer here : https://www.mozilla.org/en-US/firefox/all/ This is in case of errors download and installing with the Stub Installer.

Please read re uninstalling : http://kb.mozillazine.org/Uninstalling_Firefox and http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall

I would also recommend Updating Video Drivers https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration Try with No Hardware Acceleration also.

AM not sure how far to go as I have said nothing current that I can find and the above was how the issue was resolved before.

Please let us know if this solved your issue or if need further assistance.

Thank you for your answer 1) I have already uninstalled both programs several times (including using a registry cleaner to remove firefox references left behind after uninstall) before reinstalling. (this is not the first machine I have had to deal with this error on, but in all other cases uninstall and installing current version from full file has fixed issue)

2) When reinstalling I used the full version not stub (have found in past stub mechanism can be flaky especially on slow internet connections) As a precaution against possible download problems on his slow internet I downloaded the Full versions on to a usb memory stick from my Own windows 10 laptop with High speed internet connection. On one occasion just in case I disabled firewall and av software , so as to make sure they were not interfering.

3) To implement no hardware acceleration requires the program to run enough to change settings, the Cannot load XPCOM error is the only sign that firefox has tried to run, you can't get to the settings so therefore cannot change setting

4) I will try the Windows SFC , but concidering the program was running (and had been for at least 11 months) on windows 10 until Firefox did an auto update, and the older version of the software works ok, apart from the fact that it does an automatic update even when set to not check for or install updates!! XPCOM is not a windows code file it is a mozilla one, so SFC would not be able to deal with that issue.

Does anyone know which file Firefox is using for the XPCOM code? if I knew that I could at least check for it being present. Have looked on my working firefox for files with XPCOM in name but none exist!

Judging by the number of articles about 'cannot load XPCOM' on internet it is a very common error, yet there is very little on what causes it, and apart from uninstall software and try again, there appear to be no real solutions to the problem!

If you use Spybot, make sure to disable the Spybot TeaTimer feature as suggested here.

If you don’t, I would suggest to

1. Empty the Windows Temp folder first, usually residing in C:\Users\<user>\AppData\Local\Temp\
2. If you can’t remove all content there, note what can’t be removed and reboot
3. Try clearing the folder again, and if it still fails, run a malware/spyware tool such as the ones listed below, and then retry
4. After you have cleared the Temp folder successfully, retry the Firefox update
5. If this doesn’t help, disable or even (temporary) uninstall any installed anti-virus software and reboot, or reboot Windows with networking enabled and see if Firefox updates succeed. Such software may cause similar locking of some files, without going into details.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• Anti-Rootkit Utility - TDSSKiller
• AdwCleaner
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista if you don't already have one. Windows 8/10 have antivirus protection built-in.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Did this fix your problems? Please report back and if so, what step did.

An error message about XPCOM missing or about platform version usually happens because of a failed Firefox update that left you with a mixture of old and updated files in the Firefox program folder. This can happen in case security software was keeping a handle of some of the files in the Firefox program folder preventing Firefox from replacing all the files. You can do a clean reinstall to ensure that all files are replaced. You may have to disable security software temporarily in case the problem persists.

• https://www.mozilla.org/en-US/firefox/all/