Description

The latest release 20.9.3 that rolled into my Firefox Developer Edition ( v81.0b5 ) on an Ubuntu 20.04.1 LTS machine earlier today started hogging all of my system resources whenever a github page was loaded.

Steps to Reproduce

Optional : create a fresh profile on Firefox

1. Install Refined Github extension from addons.mozilla.org
2. Open a Github page

Expected Behavior

The page loads without issues.

Actual Behaviour

1. Firefox warns that the extension is slowing the page down.
   
2. CPU hits 100% ( on all cores ) and the fans start spinning very loudly
3. RAM starts to fill up until none is left.

Clicking the 'Stop it' button in the browser warning does not take immediate effect because the whole browser process is in some kind of a hung state. The only 2 ways ( that I could figure was ) to stop it is to either disable the add-on from the Add-ons Manager ( if the tab is already open; new tabs don't load the pages within the DOM ) or to kill the whole browser process.

Test Environment

Operating System : Ubuntu 20.04.1 LTS
Browser Version : Firefox 80.0, Firefox Developer Edition ( 81.0b5 )
Extension Version : 20.9.3