Every marginally legitimate non-profit in the US that solicits and gets donations has restricted funds. Money from that fund cannot go to anything other than what those moneys were received for.
Non-profits hate that because that money cannot be used for G&A, their other pet projects, feel good stuff or say charitable events used for fundraising (you know, the fabulous parties that non-profits throw - spend $300k to raise $303k while important people at the non-profit have a good time being photographed). Make sure to donate to restricted funds. In that case your funds will either be returned to you ( your donation rejected ) or it will be used with the restrictions you put on it.
As far as I can tell Mozilla does not have a spot to select what the donations go to. I haven't gone past the first screen on the donation page so maybe its further down the process.
They do have the option to send a check so you could restrict it for only Firefox development but any money that is not restricted is fungible. This means if I donate $100 for Firefox development they can take another $100 (that is not restricted) that they would have used on Firefox on something else. Most people do not mark their donations as restricted so I would still for all intents and purposes be funding something other than Firefox.
Non-profits hate that because that money cannot be used for G&A, their other pet projects, feel good stuff or say charitable events used for fundraising (you know, the fabulous parties that non-profits throw - spend $300k to raise $303k while important people at the non-profit have a good time being photographed). Make sure to donate to restricted funds. In that case your funds will either be returned to you ( your donation rejected ) or it will be used with the restrictions you put on it.
For the downvoters:
https://www.501c3.org/kb/what-are-restricted-funds/#