Also, Mozilla has had revenue from other search providers, like Yahoo:

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.)