This has been going on for few months and it doesn't seem to be going away (in fact, I see more complaints about similar issue around the net), so here it goes.
Up until Firefox 87 with Widevine 4.10.1582.2 I had no problems with Netflix. Then Firefox 88 with Widevine 4.10.2209.1 was released and every time I watch an episode of a show the screen freezes for several seconds while sound goes on. If you rewind and reach that point again, it doesn't happen again. However it's inredibly annoying when it happens at the climax of every episode.
I stopped updating Firefox beyond 87, but then Widevine 4.10.1582.2 stopped being supported and had to update or not being able to watch Netflix at all. While I can still watch movies, I had to start using Edge to watch tv shows (and I really don't want to do that).
I haven't noticed it happening on a movie, possibly because the last few minutes are credits which I don't watch. But there's a possibility it only happens on episodes. This is only my suspicion, but it happens right after the episode finishes buffering, so maybe it has something do with that or maybe some process with next episode is beginning while the current one is still being watched.
I'm not exactly sure it's something you can fix on your end since it seems to be involving Widevine, but would be great if something could be done.
I get around this by downloading the clip and watch it when I want.