all very interesting comments; thanks, gentlemen!
I loaded Debian from the Stretch+non-free dist. Opera is listed in the package manager although if I recall correctly it was already installed with the MINT/Cinnamon package. It does receive regular updates.
As I'm just a Debian enthusiast I try to follow the general guide -- and stick to packages that are in the package manager.
What I need to do is to learn to edit the Firejail profile and re-establish my browsing habits to work from the Firejail.
Even so, If I do sudo and install this ffmpeg.so into the extralib for Opera -- that code would then be generally accessible to the system,... While I have the download,....... it's not signed.
I think github is generally regarded as a reliable resource; the download should be OK. Still, I'd be a little happier if the package was signed.