LibreJS fails to block nonfree scripts on pages served through other protocols than http(s)

[Wojciech Kosior]

LibreJS version: 7.20.2

Browser version: Parabola's Iceweasel 81.0.2-1

Actually affected versions: all LibreJS versions for Firefox Quantum (60
onwards) and its derivatives

Steps to reproduce:
1. Save any page together with nonfree scripts to an html file. You can
also write one from scratch or use the one I trained drag&drop on (added
as an attachment).
2. Open the file in Your browser with LibreJS enabled (put
file:///<path-to-html-file> in the URL box).
3. You can also try accessing ftp:// URLs or other ones.

Expected behaviour: LibreJS blocks the javascript in that html file or
marks it as free or marks it as trivial.

Actual behaviour: Scripts happily execute and LibreJS doesn't even know
about them.

Reason: LibreJS relies solely on the WebRequest API to block scripts and
the API only works for HTTP(S).

Workaround: Use NoScript or some other extension.

My remarks:
The developers must have deliberately omitted a crucial functionality
from the extension. That's not the worst, however. A bigger problem is
that the entire concept behind LibreJS is flawed. But this mail is not
the right place to paste my essay about that.