Right now, the extension kicks in once the webpage has finished loading, but I don't see any reason it couldn't kick in at load time, so I'll test that and push a new update if it works correctly that way. Thank you for the suggestion!

Edit: Oops, seems like I was wrong. According to the Firefox documentation on WebExtensions, CSS resources (like the one this extensions loads) are injected as soon as possible ("before the DOM is loaded"), so I'm afraid I could do nothing to fix the delay you are observing, :/

(Source: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/content_scripts#css)