This is assuming that their funding is dependent on their share of browser use, or that Mozilla would get bored of Firefox if it stays at a lower share, and move on to other projects.
These assumptions would be true of a for-profit entity like Google, Apple, Microsoft, but it's not as directly applicable to Mozilla.
>>Lalaba+(OP)
I think the first assumption holds. There is a (conspiracy?) theory that Google's funding of Mozilla is based on a desire to avoid antitrust scrutiny. If Firefox usage continues to tumble, it ceases to be a meaningful competitor and the funding might be safely discontinued without legal consequences.
Mozilla exists as it does today entirely due to Google's largess.