You won't be able to work for any airline if you have hard work days at another job. There is simply absolutely zero negotiation for schedule as it's all seniority driven.

If you have total schedule control at the fire dept, ie can pick your work days AFTER you get your airlines schedule (late in the month prior) then it might barely work. But as a junior regional pilot you might only get 10-11 days off, so you would be gone every day of the month for one job or the other. Also... if you have a rough night at the fire dept and then have a long day at the airline you'd be impaired as a pilot, and in violation of FAA regs.

You can't call in sick or fatigued on a regular basis because you have another job. They will find out.

You could get some kind of part-time or contract 91/135 pilot work to build time but if you wait 8 years for the majors you'll likely miss the hiring wave, at which point the airline industry will probably be back to it's usual labor surplus.