There is a band of blue that flies across. I think that can be disabled using the animation setting: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button accepting the risk. (2) In the search box in the page, type or paste '''cosm''' and pause while the list is filtered (3) Double-click the '''toolkit.cosmeticAnimations.enabled''' preference to switch the value from true to false - - - - If you prefer a style rule change so you can retain other cosmetic animations (I actually don't know which ones they are), I have a rule for that in the editor as the sample rule, on the following page: https://www.userchrome.org/download-userchrome-css.html Does that help?