In 2021 they had 600m in revenue and spent 199m on software development. I am not sure if they break software development out to Firefox and all the other things (eg. Pocket).
So at least in 2021 at best it was 30% of revenue on developing Firefox. They spent 30m on marketing and branding.
total expenses are 340M, 200M for software dev, 27M for other program services (everyone else working on software but who's not a developer?), 30M for marketing, 81M for "general and administrative"
They have 370M in bank if I read correctly. And further 680M worth of various financial instruments like securities etc.
Does Mozilla breakdown what % of Google revenue is going towards Firefox development?