If you set Firefox for Android to clear private data when you exit, a new Quit item is added to the bottom of the menu. You need to use that Quit menu item to trigger the clearing.

Simply swiping Firefox for Android off the recent apps list doesn't trigger it because Android could unload Firefox any time for its own reasons and the developers figured you wouldn't want your data cleared by accident.

If you use Quit, is the history cleared as expected?

It simply does not work as you described. A new Quit item is not added at the end of the menus. This is ANDROID.

Hmm, I don't know why it's not working for you. It works for me. I have attached screenshots.