cpeterso got the idea from other Mozilla employees. The "he as offered to stay as CEO" grew legs when employees started responding on the governance newsgroup to people complaining about the firing. One person (Yoric I think) said he was offered a position. Then it grew to offered to stay as CEO. Then it grew to "Brendan even got angry about being asked to stay on" or some such statement. It's a classic case of incremental additions to an originally incorrect statement.

Brendan set the record straight and after that the official Mozilla line was clarified. The FAQ was updated to be clearer after Brendan made those comments.