hi AdamQK, in case you're an avast user, please try to disable https scanning within avast:

1. Open the Avast dashboard on the affected system.
2. Select Settings from the left sidebar menu.
3. Switch to Active Protection.
4. Click on Customize next to Web Shield.
5. Uncheck the "Enable HTTPS Scanning" option and click ok.

http://www.ghacks.net/2014/10/31/avasts-https-scanning-interferes-with-firefox-and-other-programs/

AdamQK said

Firefox is denying me access to Facebook, as well as my Amazon account, as it says it can't trust the certificate. I've tried doing what the forum says, going to "Add Security Exception", and pasting the FB address in, but it just says it can't get the certificate. This has only happened recently, since I upgraded to Windows 10. Could that have anything to do with it? It's supremely frustrating and, having used Firefox for many years, I'm ready to go to Chrome or even....Microsoft's new browser.

Thanks - that's worked. What a relief!