Update:

https sites *do* work in 'Safe Mode', but *not* if I manually disable the two Extensions that I'm using, which are: https://addons.mozilla.org/en-US/firefox/addon/text-contrast-for-dark-themes and https://addons.mozilla.org/en-US/firefox/addon/tree-style-tab

You can create a new profile to test if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Thanks cor-el, so far that seems to have worked, but the issue didn't show up right away last time either, only the next day after I had first installed 57.

I created a new profile and copied the bookmarksbackup folder and the content-prefs, formhistory, permissions, and places sqlite files only.

I had previously tried forcing firefox to recreate cert8.db by hiding the existing one, and that hadn't worked.

I've re-installed the addons that I mentioned in the OP, and so far, so good!

I'll update here if the problem comes back.

A couple of days later, and everything is still working fine. Thanks again!