Thanks Jindra for your answer!.
I'm sure
that the hackish solution works ok.
But (in my experience),
hackish solutions
could break something else,
now or in the future.
Things change...
I'd rather be cautious 
FiraCode is a super-nice font,
but I think I'll skip it for the time being,
until the Rstudio people fix the problem natively.
Hope they read this thread
and do something about it.
btw:
FiraCode used to work perfectly
under Ubuntu 18.04, latest Rstudio and R 3.6.3!...
Again, thanks for your suggestion!.
Really appreciated.
SFdude
Ubuntu 20.04.4,
R 4.0
and Rstudio 1.3.1056 (latest).