I figured it out. I went into the Firefox settings and told it to ask for which player I want to use (even though the default was set at VLC). Then when it asked me for which player to use at the next video launch I set it for VLC and checked the box so that it'll launch VLC automatically. I had to do this on 2 PC's so far. Don't know why Firefox launched some other video player when VLC was set as the default. Another one of life's little mysteries........
Firefox just did yet another update a few days ago, in fact two of them fairly close together. Perhaps something got reset in your options automatically.