Did this problem start recently in Firefox 140? Firefox 140 enabled support for the CSS Custom Highlight standard. Its possible that Github is detecting this change and running code that has only been tested in Chrome/Edge, and there is a glitch with it in Firefox. As a test, you could switch off CSS Custom Highlight in Firefox? Here's how: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button accepting the risk. ''More info on about:config:'' [[Configuration Editor for Firefox]]. ''The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.'' (2) In the search box in the page, type or paste '''dom.customHighlightAPI.enabled''' and pause while the list is filtered (3) Double-click the '''dom.customHighlightAPI.enabled''' preference to switch the value from true to false