Hello,

I can not reproduce this error with uBlock blocking on nightmare mode. Looking at your extensions, one seems to be a content blocker. Please try using Firefox in troubleshoot mode (instructions below) in order to see if this issue is caused by an extension or setting.

Try Firefox Troubleshoot Mode to see if the problem goes away. Firefox Troubleshoot Mode (previously known as Firefox Safe Mode) is a special diagnostic mode that temporarily turns off hardware acceleration, disables add-ons (extensions and themes) and resets some settings. See the Diagnose Firefox issues using Troubleshoot Mode article for details.

If Firefox is open, you can restart in Firefox Troubleshoot Mode from the Help menu:

• Click the menu button , click Help, select Troubleshoot Mode and click Restart in the "Restart Firefox in Troubleshoot Mode?" dialog that opens.

If Firefox is not running, you can start Firefox in Troubleshoot Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Troubleshoot Mode window appears, select "Open".

If the issue is not present in Firefox Troubleshoot Mode, your problem is probably caused by an extension, theme or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Troubleshoot Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again. (If you can't exit Firefox Troubleshoot Mode, please see Firefox is stuck in Troubleshoot Mode).

When you figure out what's causing your issues, please let us know. It might help others with the same problem.

I did not try the Firefox Troubleshoot mode first. But, when you mentioned extensions, I did try disabling a few until I found the bad one. It was Malwarebytes Browser Guard that was the bad one. Once I disabled just that, it worked again. Enabling it caused the problem. Thank you.

If you want an extension that functions similarly, please look at uBlock Origin. It has the same features, but with a different level of polish. Also, on the default mode it very rarely breaks websites.