Mozilla needs funding. By taking money from Google and DuckDuckGo specifically for search it allows Firefox to remain independent and the software it produces is underpins lots of other even more independent privacy respecting software.
The eco system around Firefox needs Firefox to survive. Unless a better funding source comes along Firefox would be in jeopardy. Having. Said that Thunderbird has been successfully turned around due to a well run community pursuing donations and volunteers.
It would also be good if countries stumped up some of the funding Mozilla and other crucial open source projects like Linux need, to maintain a strong software ecosystem. Similar to how many European countries fund national broadcasters to maintain media diversity.