What is "real" privacy.
There is no small amount of telemetry by default in Firefox, far more than Chrome. Unless one changes each of these settings, Mozilla collects data about Firefox users. Defaults can only be changed by editing the source and compiling Firefox. Almost no one does this.
"One can change settings but one cannot change the default unless edit source and compile. Almost no one does this."
There are some popular Firefox forks that have several improvements over security and privacy. Check LibreWolf and Waterfox for example.
But what if the computer user does not care to share information with Mozilla.
Information such as:
(a) IP address
(b) the software is used/when the user is online
(c) the user's preferred websites
(d) how the software is used
(e) settings the user changes or does not change
and so on.
This is information sharing with Mozilla by default. It is not "opt-in". It is "op-out".
Is having control useful for "real privacy". Is it useful for data collection and surveillance. Who should have control.
Firefox has over 37 million lines of code. The compiled program requires hundreds of megabytes of storage space. Based on LOC and binary size it is larger than the Linux kernel. Compiling Firefox on "average" computers will take hours and will require a powerful computer. It is not an easy process and can easily fail due to its numerous dependencies.
I use a text-only browser that compiles in less than a minute on an older, underpowered computer to a 1.2 MiB static binary. There are no mandatory dependencies. It is from this browser that I type and submit this reply.
Of course, that is exactly the model used by Mozilla. It exports search query data to Google for capture, by default, in return for payment.
"Thank goodness for Mozilla and all its collaborators."
Indeed, thank goodness for long-time Mozilla collaborator Google, LLC. Mozilla employees left to join Google beginning in 2005. They wrote Chrome.
Without Google's payments to Mozilla, who knows what might happen.