I was going to say that they aren't financed by Google, there's just a search contract, but I found this tidbit online...

"Historically, search engine royalties have been the main revenue driver for Mozilla. Back in 2014, the last year of the Google deal, that agreement brought in $323 million of the foundation’s $330 million in total revenue."

The default search contract went to Yahoo between 2014 and 2017 then back to Google after that. Looks like they do get most of their money from Google.