"the company looked at the history of social media over the past decade and didn’t like what it saw… existing companies that are only model motivated by profit and just insane user growth, and are willing to tolerate and amplify really toxic content because it looks like engagement… "
Those donations cannot be used for Firefox development due to the structure of Mozilla.
I'm not entirely sure that's true. The money goes to Mozilla and Mozilla will use it to fund Firefox (and other projects). It seems to work exactly how one would expect it to work - you just can't donate directly to a project such as Firefox.
There are limits to how much money they can move to projects due to their structure as a 501©3, (but all of their projects are towards an open web) so maybe not all of the funding goes to Firefox, but it still does go to Firefox.
Firefox is part of the for-profit Mozilla Corporation. Donations go to the nonprofit Mozilla Foundation. Even though Mozilla Corporation is owned by Mozilla Foundation, donations cannot be transferred to it since it is still legally a for-profit business. The funds donated to Mozilla Foundation are used for advocacy work.