When you are in Full Screen mode you can right-click empty area on the Tab bar to check if [x] "Hide Toolbars" has a checkmark.

Thanks. I don't see that item from r-clicking any empty part of any bar.

Would you please check this :

Type in the address bar :   about:config (promise to be careful. if asked) Type and look for the preference : browser.fullscreen.autohide

Its value should be set to   true   (default)

It's set to true. Thank you

Could you test in Firefox's Safe Mode? In Safe Mode, Firefox temporarily deactivates extensions, custom style rule files (userChrome.css, userContent.css), hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement? (More info: Diagnose Firefox issues using Troubleshoot Mode)

Are you sure you are in Full Screen mode and do not have the Firefox window maximized?

Yes, I'm sure. Whether I attempt fullscreen via f11, or, eg, from a Youtube video, the effect is as though I'd only maximized the window (except that there are no borders below the level of the toolbars). Oh, and there's a scrollbar. This was not the case until I updated the browser recently.

Have you tried Safe Mode, like jscher2000 suggested  ?

jscher2000 said

Could you test in Firefox's Safe Mode? In Safe Mode, Firefox temporarily deactivates extensions, custom style rule files (userChrome.css, userContent.css), hardware acceleration, and some other advanced features to help you assess whether these are causing the problem. If Firefox is not running: Hold down the Shift key when starting Firefox. If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart. Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh). Any improvement? (More info: Diagnose Firefox issues using Troubleshoot Mode)

Well, that's interesting. That works. Thanks for that! So I guess I start eliminating the variables among my extensions?

CHUD said

Well, that's interesting. That works. Thanks for that! So I guess I start eliminating the variables among my extensions?

Yes, you re-enable them one by one, till you find the culprit. But it could also be something else (like a theme, hardware acceleration).

Glad it works - would you be a dear and mark jscher2000's post as 'Chosen Solution'  ? That way others will know and might benefit from it.