Why do you think it's 0%? I doubt it, it's not like the money goes into separate buckets and engineering salaries for Firefox only come from the Google bucket, and donations get spent on lobster and champagne parties for the C-levels.
The money flow between the for-profit that develops Firefox and the non-profit foundation is one-way: From for-profit to non-profit. This is because it is illegal for the non-profit to give money to the for-profit.
Any donations you make go to the non-profit. They are not used and cannot legally be used for Firefox development.
Edit to add: Mozilla chose this legal structure. Mozilla chooses to disallow donations or payments directly to the for-profit. For example, nothing prevents using a shareware model, where Firefox is free but you can choose to pay for it. And Mozilla chooses to avoid mentioning this structure when accepting donations from you.
Firefox private donations amount to 8 million usd from the last published data. Their CEO makes 9 million a year. 0 of Mozilla donations directly make it to Firefox developers or activities.
