Note that Firefox for me within the last 3/4 months seems to have (I assume it's by design?) changed to automatically add a bookmark to the most-recently-added bookmark folder WITHOUT asking which folder to use (it just adds a blue star in the address bar) which is pretty annoying IMO and isn't the behaviour I want 95% of the time.

Pressing Ctrl/Cmd-D a second time then does "Edit bookmark", and allows you to change it.

Might that be contributing to what you're seeing? (blue star appearing is fairly obvious though).