Tried on Opera Private with my account, video also can't fast forward, clear temp/cache won't help. On another pc same issue too. (clear browser cache windows temp won't help at all)
login wife Facebook account, video no problem fast forward/backward.
BUT...facebook video play, click pause then drag fast forward/backward nav bar, click play again it wouldn't stuck.
Also, opera on android 6 my facebook account video fast forward/backward no problem.