Actually, the beard depends on the type of mask.

"Gas masks" work by drawing air through a filter. These need a fit, or "bad" air gets in too via the gaps in the face.

the SCBA worn by firefighters is positive-pressure. theoretically facial hair shouldn't be too much of a problem, as the air blows when a person inhales - and should blow out any smoke creeping in. That being said, fit IS important... it's just there is a different mechanism of avoiding toxins. Recall that a SCBA technically is a level of chemical-exposure protection.