Does it still happen in Troubleshoot Mode?

@zeroknight: Hard to tell, because a restart usually resolves the problem on its own, and switching to Troubleshoot Mode does require a restart so not sure if it's the restart that resolves it or the Troubleshoot mode.

What I did try is to disable all extension (I have about 7). And this did not resolve the problem.

You could try creating a new profile for Firefox. In the first article linked below there are sections telling you how to open the profile manager with Firefox open or closed.

https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles

See the other articles for copying data from the old profile to the new profile. You need to do that with Firefox closed.

https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data#w_finding-your-profile-without-opening-firefox

Does it still happen if you disable hardware acceleration?

@Terry: I have now done a "Refresh Firefox" (the option you get when you restart in Troubleshoot Mode). Just did it right now so I will report back if this resolves the problem.

@zeroknight: I tried this and it didn't solve the problem.

Can't you close that tab via a middle-click or via the right-click context menu ?

Do you have any third-party antivirus or security software?

Does it still happen if you reboot in Windows Safe Mode with networking enabled?

@ cor-el: I tried closing the affected tab and opening a new one, but the new one is also unresponsive.

@zeroknight: as this only happens sporadically, I cannot test this as I cannot spend an entire day in safe mode with networking disabled. Also, if networking is disabled, it would be difficult to test whether the tab is working as I won't be able to browse the internet.

With the Firefox Refresh it has not returned so far (24h). Maybe this solved the problem. If not I will post again here. Thanks for all your responses.

There is an option to have networking enabled when starting in Windows Safe Mode.

@zeroknight. Sorry, I had misread your message as "networking disabled". I'll give it a try should the problem reappear.

I can report that the problem persists also in Safe Mode. I suspect that it is a memory issue. Right now I have a single Firefox window with 20 tabs and in the Windows task manager I am counting an incredible 18 instances of Firefox using a combined RAM of about 22 GB. One is showing under "Apps" as "Firefox (13)" and the other 17 are under "Background processes". So it seems that the dead tab issue and load delays I am seeing sometimes are in fact memory issues as Firefox seems to be a tremendous memory monster.

I am not sure if I should file this as a bug report but since I am only speculating about the cause, it probably won't be looked at. However, it massively affects the performance of Firefox in my day-to-day usage (also on my wife's computer) and we are both thinking to switch to another browser, although I would be very reluctant to do so as I am on FF since the Netscape days.

18 instances of Firefox using a combined RAM of about 22 GB

You can get more details by visiting about:processes in the address bar (Shift+Esc).