Stuart Ellis (former CEO) used to say family first, job second, CFS third.
I hope you find a job, Sternzee, that suits your CFS commitments. You don't really want to be in a position of applying for jobs and being asked why you left your last three or four employers if the answer is, CFS!
Many employers are simply in a position where they cannot allow employees to attend. For example I worked in a kindy where the student/staff ratio simply didn't allow me to leave even though I was entitled to emergency services leave (it just required notice).
Small businesses can struggle here too. Even if they don't pay you for your time off, they have to pay someone else to do your job or suffer the productivity (ie money) loss.
Although I think it's good to raise the issue of CFS commitments in an interview and see where your prospective employer stands, I think it's best to go gentle to begin with. Get the job, be invaluable, and then do the CFS thing
Also if you show a willingness to make up time, do not go to every job going that others can handle, communicate your absences and provide written evidence of attendance from your Brigade it can help.
The EBA my workplace just negotiated has, for the first time, formalised their policy of supporting volunteer emergency service attendance by employees. They have always paid us for such time off but now it's in black and white!