Are you talking about clips you uploaded or downloaded? Please provide public links to these clips.

Is it this:

You see the video buffering and there's a pause button, but no video plays, and then if you click the pause button and then the play button, THEN the video plays?

Those symptoms can be caused by a hidden setting. You can switch it here:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste autop and pause while the list is filtered

(3) Double-click the media.autoplay.enabled preference to switch it from false back to its default setting of true