Im one of those die hard firefox users and have been using it for over a decade.
It’s quite worrying that it relies on Google to exist. This article details the finances behind it, and it’s an interesting read.
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Mozilla have tons of money, and the donations are tiny in comparison to their real revenue stream.
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70% of revenue comes from Google, by having Google Search as a default search engine.
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Its not clear where the revenue is being spent. There are no large costs.
 



I for one was not familiar with this and even if the source is not reliable there sure is an unhealthy amount of politics involved.