Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

Are you using any blocking programs / add-ons?

The problem disappears in Safe Mode, thanks so much.

Yes, I'm using various blockers and other add-ons besides, though no new ones since the Firefox 42.0 update.

I'll work through them one at a time to identify the culprit.

Thanks again!

Once you think you found the problem, disable that and re-enable all the others, then restart again. Let us know who the suspect is detective.

It seems HTTPS-Everywhere was the culprit. Maybe if I just upgrade my OS X from 10.6.8 that'll deal with it too. Thanks again Fred.

That was very good work. Well Done. Please flag your last post as Solved Problem so others will know.