My biggest issue with FF mobile (android) is sometimes when you open a link from a different app, it opens in an existing tab navigating away from a page you already had open. It should always open a new tab.

So I might open a link in my email app, then when I tap the back button on the phone, the expected behaviour is it will close the tab and return to the email app. Instead I'm finding it navigating back to the page that was open previously on the tab that was already open.