Bookmarks and history if you didn't backup would be gone especially if the old FF was x86 the O/S was x64 then it would install FFx64 not a x86 version and it most likely removed all previous FF that is why there is nothing that can be found. You can make FF your default Browser when installed or changed in the settings.

Thanks for your response.

I am not quite sure what you mean but it it helps, FF was Quantum . System is 64bit. Despite trying to make FF default, only option offered seems to be Edge or Explorer. Incidentally the update also removed several programmes as weel as the Icons.

When you updated to Windows 10 it can and will do anything it wants including uninstalling working programs it just does not happen to like at the time.

It is possibly when Firefox Updated you have 2 Profiles. https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data if have to it is possible to merge some if not corrupted.

If this has been less than a month and you have not used a CCleaner or another like Microsoft Disk Cleaner  : Clean Up System Files you can revert back to your Previous OS. Can not say if Firefox will be as you left it or not. If is then do this all the time. Copy over top and will have them all the time. Mine are from 95. https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

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

Sorry about delay in getting back to you. The problem was the update and MS support eventually admitted same but had to perform a clean install - the telephone support went on continuously for 5 hours. They also tried to sort out my PC which had startup problems and freezing - ending up messing it up and now can't even start machine up Thank you Microsoft! They then said they would phone again the next day. The (new) technician who said he was a Supervisor, downloaded a clean Win 10 to a USB via laptop but couldn't work out how to install on PC - can you believe it? Anyway, they asked me to contact HP to ascertain BIOS settings and said they would phone back in 1/2 hour. That was yesterday at 4.30pm I am still waiting! So now into 3rd day without PC. Incidentally, the guy at HP says he constantly gets problems with Win 10 updates! Trouble is, with Microsoft it appears no one is accountable.