Information about mandatory firings can be found in the WTB Rules section. You can find the link to them at the top of the WTB page. Under the subtitle of "Voting" you can find what leads to mandatory firings:
In each round of the game, the employees will cast votes for the Boss. The Boss and the Rat do not vote. The voting deadline will be clearly indicated. If you do not vote by the deadline, you will be instantly fired due to poor work performance. You may change your vote at any time up to the deadline.
It implies mandatory firing.