I'm told that this is incorrect and that the FSF has been doing it for 34 years, the EFF does it, the ACLU has done it and others have even paid for proprietary software development (their apps etc).
i worked for mozilla, your donations pretty much do not go to coding firefox. there are side effects where it will affect code, or someone that gets employed by foundation that will now have money and free time to code for firefox for free, but its not like "your dollars go to hiring a sw engineer".
the donations are still useful though because if there were none mozilla foundation would die, and in turn, mozilla corporation would also die or be sold as its fully owned by the foundation
