Thunderbird, ligatures, and "fixed-width" fonts

I read mail with Thunderbird and for my fixed-width font I used Noto
Mono.  Since by design Noto has glyphs for all Unicode characters,
that means it contains glyphs for ligatures (those character pairs
beloved of typographers that condense certain combinations such as
"fi" and "ffl" into single glyphs.  At some level between the text of
the mail I'm reading or composing, which does not use those Unicode
ligature code points, and the pixels that appear on my screen, my
fixed-width text is no longer rendered with fixed width glyphs.

Google found me this harfbuzz issue:

https://github.com/harfbuzz/harfbuzz/issues/4414

However, a search in the source of the current versions of pango,
harfbuzz, and thunderbird does not find any occurrence of "no_liga",
the option mention in the above issue.  So I'm at a loss as to how
to debug this.  My desired outcome, of course, is for the characters
in my plain text emails to be rendered on the screen in fixed width.
Any suggestions?                                         -- George

P.S. I have a new unexpired PGP key:
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