We already request permission on all `http://` and `https://` hosts.
That should give us all the permission we need. The `activeTab`
permission just seems redundant. It's probably a holdover that no longer
applies.

On Firefox, when adding or modifying CSS files, the changes aren't
applied on a page refresh. Instead, they're applied when the extension
is reloaded. Provide a keyboard shortcut to enable quick reloads, rather
than having to open `about:addons` and toggle the extension off and on.

This variable is already declared at the top of the loop. We don't need
to redeclare it in the if/else branches.

Firefox has been my primary browser for several years now.