Firefox

How exactly could it?

The code is fully open source. No premade binary blobs in it that we can't investigate like in chromium.

At worst, Mozilla could change the license so that you aren't allowed to fork their code to remove the data collection features, and the community could either tell them to get bent and do it anyway, or just manually backport stuff from post-license change to a codebase built off pre-license change code and be in the clear.

But they aren't going to change the license. Usually that would require them to get permission to do so from everyone who contributed any code currently in use by the project.