Initially, after checking the option to clear private data on quit/exit, it worked ... sort of. Just sometimes. Firefox for android seems to lose this setting, although the option remains checked. IMHO, this is a fundamental flaw, one likely to provide false confidence that data has been cleared, when it hasn't. A solution would be to fix it or bring forward the clear data trigger to the same menu as the quit button. As a programmer, I have tested all fundamental basics, in case you were wondering. I'm running v49 for android.(now on version 50.0.2 - same issue persists)