I've had this problem for about two years now. Can't the fix it? Vivaldi solved it somehow.
Every time I update, I start looking for the damn libffmpeg.so file.
For Opera 79.0 install latest Vivaldi browser 4.2 and the take the lib:
sudo cp /var/opt/vivaldi/media-codecs-79b56700ea9558c50a5edacaeb70298120127d2f8b32bb5e05c3876a51bd64b5/libffmpeg.so /usr/lib/x86_64-linux-gnu/opera/libffmpeg.so