What OS?

Thank you, i put the OS in the original post but it does not seem to have survived. Windows 10 Pro x 64, Version 22H2 (standard auto update from Windows)

What security software are you running? Where are you getting Firefox from? Can you see if Firefox works normally in Window Sandbox. https://www.howtogeek.com/399290/how-to-use-windows-10s-new-sandbox-to-safely-test-apps/

Got firefox from https://www.mozilla.org/en-GB/firefox/download/thanks/ Security system = Malwarebytes (have used it for years with no problems, including with Firefox) Will try to use sandbox, with the disclaimer that I am basically a very basic click-the-button computer user, but happy to try.

You should disable Malwarebytes, reboot the computer and try again. Just because you've been using something for years, doesn't mean that it won't break one day.  ;-)

Treied to download Firefox from C-net, on trying to install, it said there was a dll file missing (see image)- but no hint given as how to solve the problem. ????????????????

Try this.

https://support.mozilla.org/en-US/questions/1342629

OK, I downloaded the runtime library from Microsoft that a search query told me was commonly missing, then reinstalled Firefox via C-Net (although it is apparently an old version and i now need to find my old bookmerks somehow) and it works. Whoopee! Thank you for all the help and encouragement, and i hereby nominate myself a computer genuis, Grade Z.