(I am a former Mozilla employee who was there when the layoffs happened)
This is what I know about what happened there:
There were two enterprise IT teams with similar duties but different purviews. When management was deciding on layoffs, they decided to unify those two teams. Unfortunately that meant that there were redundancies.
My heart goes out to those who lost their jobs, and they have every right to be upset.
But the inferences being made as a result of the resulting tweets just weren't true: this notion that all security teams were wiped out is false. And there are now others assigned to threat management.
Furthermore, the security teams that work on Gecko and Firefox were left mostly if not entirely intact.