Mozilla does not need more money than it already has. Ladybird has shown that you don't need a huge team to build a browser from scratch, so clearly Mozilla isn't spending all the money it gets from Google for development.
Of course, it’s also good to have the option to disable it.
The problem is that the surveillance mindset has infected developers these days so now you have a bunch of telemetry apologists and advocates influencing the narrative. Unfortunately they are themselves fine with being spied on... although the inevitable leak happens and hopefully some of them start to worry.
The point of telemetry is not to estimate the number of active users, and focusing on "telemetry" is the wrong bogeyman. Things like crash reports are telemetry, and are perfectly acceptable as long as the user remains in control over whether or not to send them. Software should respect user privacy and user control.
I'm perfectly fine with telemetry in any software, free or paid. Just make it opt-in and don't turn into cookie notice style malicious compliance about that.
Don't install it if you dont want it, that is where you choice lies. Telemetry inclusion is up to the author. Good ones will make it opt-in if it is truly necessary to include.
> Ladybird has shown that you don't need a huge team to build a browser from scratch
Ladybird has done no such thing.
It has a cost and can also become a liability. You don't do it just for fun.
Ironically, it's the people who disable all telemetry out of ignorance and paranoia that end up crying every time a developer deprecates their favorite feature or they have an issue with a new release. If you don't vote you don't get to complain. Mozilla's PM's in particular have repeatedly published and cited telemetry data in their decision making, and have referenced crash reports in proactively filed bugs.
Whether or not telemetry counts as spying depends on whether or not informed consent was obtained before data is sent. If any data is collected about me, my machines, or my use of my machines, and I have not explicitly been told about and agreed to it first, then I'm being spied on.