Yes, but not really to Mozilla. The Google money is practically a subsidy, it's not really necessary.
Consider this: Firefox's remaining userbase is mostly quite technically sophisticated users. Are they going to blindly end up searching on Bing all day because they don't know how to change the search box default? No. They're going to set the default back to Google, and carry on as before, except then Mozilla don't get paid for it.
Google have a few reasons to cut them off. One is, obviously, it won't make much impact on their revenue given Firefox demographics. The second is Mozilla and Apple are dragging down the Chrome team. Chrome tries to introduce new tech, Mozilla vetos it or doesn't implement it. Apple they can't do anything about, Microsoft gave up on Edge and is now in the Chrome camp. If Mozilla dies, only Apple remains on a non-Blink engine. And eventually Tim Cook will wonder why they're funding a poor clone of Blink for legacy reasons, when they could just use it directly and re-allocate the resources to more profitable projects.
Cutting of Mozilla's lifeline is one way to push the web along to standardisation on Blink as a codebase. And when that's done, well the W3C is irrelevant for a long time already, so HTML becomes the API to Blink basically and they add features at whatever speed they want.