If the Source code is open Source, then so is the Firefox build
If that's the case, Mozilla wouldn't have spent so much publicity and gotton so much shit a year+ ago for removing the MPL from their executable and developing their TOS.
And even if it did, their TOS contains provisions that they can use your telemetry for any reason and they initially said they wouldn't use it to train AI and then they admitted that they use it to train AI. So even you give them the broadest interpretation of OSS, there are still reasons not to use Firefox executable and use an alternative instead.