The complaints were unsubstantiated with no examples of offensive behavior provided and might as well have been a smear campaign by people brigading by sending abuse reports en masse.

And unlike a private, for-profit company though,(for which your argument that they are within their rights to provide platform/deplatform whomever they want, would indeed hold) Mozilla attempts to claim the moral high ground in their manifesto[0], citing their commitment to "an internet that promotes civil discourse, human dignity, and individual expression."

[0] https://www.mozilla.org/en-US/about/manifesto/

EDIT: clarified the first paragraph slightly.