hi SharonJBlack, are you using kaspersky as your security program? if so, please disable the scanning of secure connections like it's described at https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER#w_kaspersky, reboot your system afterwards and see if the issue is still occurring...

I have Norton Security

oh ok, can you describe what exactly is happening when you try to open facebook - are there any particular error messages?

An error occurred while processing this request. Please try again later.

This has been happening on Mozilla for over a week.

thanks. can you try to clear the cache & cookies from facebook.com and see if this addresses the login issue?

I clear History, cookies and cache daily.

can you replicate this behaviour when you launch firefox in safe mode once? if not, maybe an addon is interfering here...

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

This still doesn't fix the issue.

please try to refresh firefox and see if this will make a difference.

I did that earlier and it didn't help but I will try again.

I refreshed Firefox again and still can't open Facebook.

then the last troubleshooting step that i could think of is to create a new firefox profile for testing purposes: Profile Manager - Create, remove or switch Firefox profiles