To make it work, you gotta do three things:
1. Add `document.documentElement.classList.add("fixed-header");` to the userscript (should be like one of the first things that runs, and NOT part of the code that's injected into the page.
2. Add `// @run-at document-start` to the script metadata, so that we get a shot at modifying the DOM before chat initializes
3. Re-add that bit of logic to listen for the "load" event before injecting the script. This'll now be needed by both Firefox *and* Chrome, since otherwise we'll be running before chat is ready.