No idea. I've been using Firefox and Sync for years, currently on 5 devices (one android). Never had a problem. Firefox 121.0 (64-bit). This is the "release" channel. Maybe you're beta testing?
You know what, you could be onto something. I'm using the Firefox Developer version (122.0b3 64 bit). In any case, how are the differences resolved? E.g., in my recent case, all the devices ended up with only a handful of passwords (17), whereas the Linux laptop had 858. When I brought that laptop online (after exporting them), it synced and also ended up with the 17. I wonder what the logic was to deduce that the 17 are the ones to keep and the 858 should be deleted (there are some common ones of course). Because after sync when I imported the 858 back, it kept them all and synced them to all other devices, so it considered them as additions that should be kept.
Me too. It is so flawless that I had forgotten how good it works until seeing this article pointing that fact out.
Same, I've been using Firefox for years and it's always just worked. Recently it got faster, too, now sending tabs between devices is instant.