your add on is very intresting and very good. I like it and i start to use it, for now ,,,
as you already know, sometimes the code become heavy,
in some weak devices, or
in sites that are already heavy, by inject too much js code to the user.
Because of this last, and because most of the sites, start to become like this ( i mean put the
user to build the page, and not the server => thats why our browsers seems so incapable some times,
and take the CPU/Memory and create big swaps (but this is another discussion),
i want to share some ideas.

1. there you have your tabs background / images / texts /.... ,
give an option to disable any of them. So later your js to have less work.

2. This will work, In sites that have somehow very similar CSS selectors across their pages.
instead to inject class to query selectors through JS, compile a CSS by the 1st page of the site,
and then just inject the style, after the page load. I believe in facebook, youtube this will work
quiet well, and give an option to the user (maybe in Common section) to use this mode.

sorry, for my big review. Nice work !