This extension redirects before a request is even made. `document-start` runs when the whole page is downloaded, but before the DOM is constructed, which is a lot slower. As for portability, both Firefox and Edge are committed to supporting Web Extensions, which is pretty much the Chrome extension API. This extension is not tied to any store. You can install it manually exactly as you would with a userscript. Actually, userscripts in Chrome, are just re-packaged as native extensions on install.

A greasemonkey script in Chrome would take the same amount of memory or more, as Chrome just repackages it as an extension on install.