Looking at proc maps (i.e. what files firefox is loading), it shows this.
7f2b32240000-7f2b3226b000 rw-p 00cf4000 08:03 4850011 /usr/local/lib/libavcodec.so.58.54.100
7f2b39866000-7f2b39867000 rw-p 00094000 08:03 4850020 /usr/local/lib/libavutil.so.56.31.100
7f2b4a11f000-7f2b4a122000 r–p 00000000 08:03 4850014 /usr/local/lib/libswresample.so.3.5.100
It is loading the shared libraries. The static build contains no libraries, therefore is no replacement for these instructions to build
ffmpeg to override the CL provided version.
You could try putting the files in
/usr/local/bin instead as they are binaries and ensuring that
/usr/local/bin is in the PATH variable. (Putting them in /usr/local/lib does nothing as they aren’t libraries). If firefox wants the libraries (which means the static version won’t work with the binaries in
/usr/local/bin), then ignore the static version and build it via the instructions provided.
The static version will only work where the program runs the
ffmpeg binary directly (then libraries don’t matter), while most programs will want the libraries.