Hello kameelt,

This error is probably caused by the "McAfee MSS+ NPAPI Plugin " extension that you have installed. try to keep it disabled for a while and see if the problem is still occurring..

Also try restarting Firefox in Safe Mode

Diagnose Firefox issues using Troubleshoot Mode

In safe mode there was indeed no problem. In the normal mode I have only 2 extensions running (avast antivirus and adblock), but even with these two disabled I still have the problem persisting ... . Is there anything else (besides the extensions) I can try to disable?

At the top of the Firefox window, click on the Firefox button, go over to the Help sub-menu and select Troubleshooting Information.

You will be taken to a page with the address about:support.

Copy to clipboard

Clicking on the Copy text to clipboard button will copy all the text on the page to the Windows clipboard.

After copying, you can paste the information into another window for someone to see by clicking the Edit menu in the program you are using and then selecting Paste (or by holding down the Ctrl key and pressingV).

see the images for more help

Firefox's safe mode also disables Firefox's use of hardware acceleration for graphics and the JavaScript compilers. Those could be disabled manually (but I have to run and can't type out the steps at the moment).

This looks like a problem with Avast! Online Security (wrc@avast.com)

A script on this page may be busy, or it may have stopped responding. You can stop the script now, or you can continue to see if the script will complete.

The above script message was what drove me crazy when opening Gmail. The problem was with Avast as cor-el mentioned above. Disabling the Avast online security add on solved the problem.Hope it help you too. Cheers