Hi, I just wanted to add that I'm having a similar issue. I don't use Sync, but my locally saved bookmarks keep vanishing every few days from Firefox on my IPhone, as does my browsing history (as seen that the websites I usually visit aren't even suggested). The bizarre thing is that it does remember the tabs I had open. This is getting really annoying, especially given my addiction to checking fivethirtyeight's forecast throughout the day.

I have solved this problem. After 12+ years of using Firefox even after getting my iMac in '10, I have dumped all things Mozilla. No more Firefox, Thunderbird, Firefox for iOS on both iPad and iPhone. I am getting along just fine using Safari and Apple Mail. Good luck, Tom