Bring back code needed to install uBlock

The patch we use to install uBlock was based on copy'n'paste'n'adapt the code
Tor Browser used to install HTTPS Everywhere. This code is not there anymore:
git diff tor-browser-91.10.0esr-11.0-1..tor-browser-91.12.0esr-11.5-1 --
browser/components/BrowserGlue.jsm.

Indeed, HTTPS Everywhere was removed from Tor Browser 11.5: https://gitlab.torproject.org/tpo/applications/tor-browser-build/-/issues/40527

So let's bring back some of the code from
https://gitlab.torproject.org/tpo/applications/tor-browser/-/commit/a8774c91a05bccc5e7ebe1bda0cd37aa2b4a6e64
that our patch implicitly depends on.

Closes #19059