How to end process while quiting, Firefox
The regular Firefox for Android is not a privacy-focused browser, it's a general purpose browser with (IMHO) the best privacy defaults that balances privacy and day-to-day usability, but it's not meant for high privacy.

You should use Firefox Focus instead, which is actually designed to be an incognito browser. Additionally, if you really need the highest level of privacy, you should be using Torbrowser.