The extensions page could only hand out tarballs; installing meant going
to Settings › Extensions › Community and finding the entry again. Pages
now get window.bcnr.installExtension(id) and Theseus intercepts
theseus://extensions/install/<id> links (page clicks, target=_blank and
the address bar). The page only names a catalog id: Theseus fetches the
catalog and package itself, asks in a native dialog the page cannot draw
over, verifies the publisher signature against the name's current owner
and activates the add-on — the same path a Settings install takes. One
prompt at a time; an already-installed version says so instead of
offering a no-op update.
The site shows the button inside Theseus (feature-detected on the
bridge), a "update Theseus" hint on older builds and a download hint in
other browsers.
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