At least in Google’s case it makes good business sense - as in, it already works for them and their target audience already has been using the OS with all that, and doesn’t care about it, ads and tracking; besides there aren’t too many phone OS options.
But in the case of Mozilla? Heck the target audience cares very much about these things and not liking shit like this was a reason they were in Firefox for. Now a lot have already dumped it for Safari, Brave etc. Then they keep doing such tricks.
Someone at Mozilla has to be especially dumb to have done this.
If there were a viable non-chromium alternative to Firefox I’d already have switched.