I have the same feeling now... Firefox doesn't crash anymore, even with 1000+ (unloaded) tabs... but I will often found that the page (or a youtube video) is now gone/removed after months or even years.
I suspect it still loses data and maybe corrupts profile if there is a disk full.
It deletes history entries once they're too old, and it also only keeps the last timestamp of when a URL was visited, so you can't know how often you visit a URL.
I suspect it still loses data and maybe corrupts profile if there is a disk full.