Percentage of total users is deceptive in some ways. The absolute number of FF users can stay the same, or even rise, and their % share can decline. It is absolute number of users that impacts the deals they can make. That's not to say a larger % wouldn't be good for FF.
Firefox needs a serious presence in mobile. If the browser is competitive with Chrome, and also supports webextensions, it stands a fairly good chance, at least boosting up a few percentage points.
