Just as a note, you can use the Alt key to clear a Firefox menu, which may have a lower risk of odd side effects than the Esc key.

As for why it sticks, I don't know, and if Firefox's "Safe Mode" didn't help, then perhaps Firefox's program files got corrupted during the last update? Our standard recommendation in such cases is:

Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean (no inconsistent or alien code files). As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

It only takes a few minutes.

(A) Download a fresh installer for Firefox 48.0.2 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.) Choose the "Windows" version (32-bit) rather than the 64-bit version for maximum plugin compatibility.

(B) Exit out of Firefox (if applicable).

(C) Using Windows Explorer/My Computer, rename the program folder as follows:

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(D) Run the installer you downloaded in step (A). It should automatically connect to your existing settings.

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• \OldFirefox\Plugins
• \OldFirefox\browser\plugins

Any improvement?

Thanks, tried it, but no joy, the condition persists. BTW, the Alt key has no effect on my open menus either...

As a test to see whether this could be a settings issue, you could test in a fresh Firefox profile.

New Profile Test

This takes about 3 minutes, plus the time to test sites where this problem occurs.

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Click the Create a New Profile button, then click Next. Assign a name like Sept2016, ignore the option to relocate the profile folder, and click the Finish button.

After creating the profile, scroll down to it and click the Set as default profile button below that profile, then scroll back up and click the Restart normally button.

Firefox should exit and then start up using the new profile, which will just look brand new.

Do right-click context menus work any better in the new profile?

When you are done with the experiment, open the about:profiles page again, click the Set as default profile button for your normal profile, then click the Restart normally button to get back to it.

Thanks again for the reply. I created the test profile and all the right-click actions and context menus function normally when I switched to it. They began misbehaving as soon as I reverted back to my original profile.

So, if Settings is the culprit, are there some usual suspects I could check, or am I back to blowing away the install?

I think I found it:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste popup and pause while the list is filtered

(3) If the ui.popup.disable_autohide preference is bolded and "user set" to true, double-click it to restore the default value of false

Well done, sir. That was indeed the culprit. Not sure why or how it got toggled to True, and that open question will no doubt bug the voices in my head until I end up doing a fresh install anyway, but for now, reverting it back to the default value did the trick.

Thanks! I appreciate the help and tenacity.