

Forking Firefox means it isn’t Firefox - yes, this means that the original was OSS, but you really need to be an expert to get at all the OSS code running on your machine. I mean that it is literally not Firefox, since your fork doesn’t have permission to use the trademarked name.
This is only relevant if you are planning to redistribute it after you make changes. You can make any and all changes you want to FF on your machine to remove telemetry, and you do not have to remove the branding.
If we think of the enabling functionality in Firefox as a virtual lock, breaking that lock is illegal under the DMCA. That seems very weird for code that is ostensibly open source.
Extending this argument would mean that it’s potentially illegal under DMCA to remove any protection mechanism that it would be ‘hacking’ to bypass during usage (e.g. SSL, authentication, etc) from any OSS project. Thats not the case, because an OSS license gives you explicit permission to modify the application.
I understand it’s a slippery slope argument, which is why I didn’t find it particularly convincing.
And if you’ve done bugfixing of software, you know that the data that users give you in reports is 90% of the time less useful than what you get out of crash reports or telemetry (yes, there are rockstar testers out there, but they’re the vast minority of users). Not all beta programs are there solely for the developers’ sake (some are there for e.g. third party devs to update integrations, etc), but this one seems to be, and that isn’t evidence of malice.