Fixes the extension not working at all in Firefox — the panel would open but stay blank on every
supported site. (Firefox restricts a browser API for extension pages loaded inside another site's
page; lookups now go through the extension's background page instead, which isn't affected.)

Fixes the "Filter Forge" tab occasionally disappearing after a page finished loading, on sites whose
own scripts rebuild parts of the page after the initial load.

No changes to filter generation, strictness levels, or supported sites.