Most of the questions here relate to version 100. Can you describe the problem?

Thanks Terry. As I've said, the browser just ceases abruptly after a few hours of running. I've downloaded the '100' version a few times, and even tried uninstalling the older versions each time, in case there was something 'clashing' with the same files in the new version. Can't be more techy I'm afraid.

Do you mean it stops running or that you can't do anything with it?

When the problem starts (after I've been working for a couple of hours without any hiccups) and I try to log into a fresh website, the screen comes up with something like "Hmm... There a problem with loading this page ~ Try again?" I hit "Try again?", but I can't progress beyond this. And then, even more frustratingly, when I try to go back to the website I was working on just before this problem, that website also gives me the same "Hmm... There a problem with loading this page ~ Try again?" message! And then I have to go over to another browser, find the Firefox website to download the Firefox browser all over again.....It gets me into a recycling loop!

Is that a page you accessed before during the session and tried to go back to?

There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connection certificates and send their own.

https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites

https://support.mozilla.org/en-US/kb/secure-connection-failed-firefox-did-not-connect

https://support.mozilla.org/en-US/kb/websites-dont-load-troubleshoot-and-fix-errors

What do the security warning codes mean

Thanks Fred. I don't have any of these, but I use Malwarebytes and Comodo to scan for and firewall potential threats... Could one of these have caused the problem by disabling the Firefox browser?

One way to find out is to disable one and see if the problem returns.

I'll try that....Thanks Fred