Geany stores whatever syntax it was using and never checks again […]. Stopping and starting Geany will not "fix" an existing opened file.
Isn't that the *only* real issue here? No, Geany won't change the file type of a loaded file, even on restart, so long as it's part of the session. To force redetection, the simplest solution is to close and reopen *the file*.
So if you had your .F77 file open all along, no, changing the extensions lists will not have helped. Of course, when you added .F78 I guess you loaded such a file *afterwards* and thus it worked.
Have you tried to open .F77 files you never had before, or Close each file and reopen them? I really can't figure out why .F77 in particular would give *you* trouble…