The point is that with open source you can effectively leech off of Google for now, while still retaining the flexibility to nope out and do your own thing at any point you decide.
Considering just how severely behind they are already (as I mentioned in my other comment, they’re often 3–5 years behind other browsers in implementing new web standards or operating system features), I see anything they can do to reduce how much they need to maintain independently as a good thing. In an ideal world where they had all the funding and development power they could want I might say sticking with the completely independent Firefox would be great. But that just isn’t where they’re at today.
I’ve seen two cases that actually directly impacted my ability to use Firefox. I can only presume there are many more. Those being supporting the column-span CSS property (available since 2010 in other browsers with vendor prefix, and early 2016 without, while being late 2019 for FF) and supporting iPad OS’s multi-window functionality (introduced mid 2019, Firefox has had it for just a handful of months now). I have first hand experience telling me very directly that this is true.
There’s also been a lot of talk about Firefox’s lack of support for PWAs. I’ve not experienced that myself to be able to comment more than to say I’ve noted others have complaints.