Is one allowed to be a firefighter if he might be required to work on Shabbat? or is he allowed to, in order to save lives?
When I was an EMT I lived in a place where there were many Jews in the rescue squad. The local rabbis told us to arrange a rotation which ensured that there was always someone to answer calls but that we didnt have more people than we need.
I would say that Jewish firefighters should try not to be on-call on shabbat but if they need to respond to life threatening emergencies they must.
As far as I know, since Fire-Fighting is all about rescuing lives (~ Pikuach Nefesh, פיקוח נפש) then it has priority over all other Mitzvas.
Source from Wikipedia: Here.