Numbers not displayed in the editor on Firefox

The numbers are not displayed in the editor, which causes a major issue with doing any of the challenges. I’ve noticed this in the very first challenge, and it occurs in the second one as well.

The numbers are in the value of the element when inspecting or copying the code, together with displayed on the right preview pane, but for whatever reason not displayed in the editor.

I used Firefox’s private window to rule out any extensions or cache. I have no adblocks or anything preventing Javascript, etc. Doesn’t occur on Chrome, but I prefer to use Firefox.