Then they aren't doing a very good job of it.
>Firefox makes up about 90 percent of Mozilla’s revenue, according to Muhlheim, the finance chief for the organization’s for-profit arm — which in turn helps fund the nonprofit Mozilla Foundation. About 85 percent of that revenue comes from its deal with Google, he added.
https://www.theverge.com/news/660548/firefox-google-search-r...
What people usually say is that Google merely wants Firefox to survive for anti-competitive reasons. Presumably that does not necessitate it actually being used (or be usable).