People keep repeating this but couldn't Bing replace Google as the default? The "default search engine" position is actually worth money.
https://gizmodo.com/yahoo-s-insanely-bad-deal-to-pay-mozilla...
So I don't think their position is as fragile as people suggest. The problem is declining market share, not that they are tied to Google per se. (Of course Google is by far the strongest search engine, but it's plausible that Mozilla could survive with Bing.)
In a scenario where Google no longer sees the value in that arrangement, what value would Microsoft be seeing in it?
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.
By accepting the Google cheque, they directed people towards Google and may have indirectly funded Chrome and monoculture.
If they had directed people towards Bing when they had 30% market share, that may have resulted in a search engine duopoly instead, and a greater chance of Microsoft selecting and funding Gecko/Servo instead of Blink.
But the amount it's worth is probably roughly proportional to the number of Firefox users. With Firefox having less and less market share, the attraction to search engines of supporting it -- particularly those whose operators also make their own browser -- must be diminishing rapidly as well.
The odds of them being able to build a business that that replaces the several hundred million they get from Google (which is essentially pure profit, not even revenue) are essentially nil. The instant Google decides they're no longer useful, Mozilla is going to go poof.
I'm inclined to agree with others who have said they should be building their own endowment. It's almost malpractice on the part of their leadership not to.