Since I just recently completed my certification as a Firefighter I/II (also Wildland FF 2), and have been an open source developer for more than 35 years, with the last 25 years of that being full time and the last 15 having FLOSS as my actual income source, I'd like to comment.
The primary difference between professional firefighters and their volunteer counterparts is hours on the job. When I graduated from the state academy, I knew as much as about firefighting as any of my fellow graduates and had been through precisely the same training requirements. However, the gap is going to open up very rapidly since the career guys will be doing regular shifts every week, whereas I will be answering 1-3 calls a month on average, most of which will not be fire-related. A year from now, the career guys will be even more familiar with everything we learned during our certification training and more, while I will be working hard to remember any of it.
So the question is: how well does this analogy hold for s/w development?
It doesn't.
First of all, the gap between most proprietary development outcomes and their FLOSS equivalents has more to do with UI/UX design questions than actual coding skills. At the source code level, it's generally proprietary projects that are "burning down left and right" (shoddily and quickly built, with inadequate attention to engineering and insufficient caring about one's work due to marketing deadlines).
Secondly, the difference between proprietary developers and their FLOSS equivalents in terms of hours of experience is not deterministic. It's going to be a function of employers, personalities, life situation. Plenty of (typically younger) FLOSS developers squeeze in more quality hours on their FLOSS work than their proprietary cousins do.
Thirdly, a firefighter only gets to put out the fires that actually happen. A software developer can pick their own problems and goals and work on them at any time. There's no relationship between the outside world and your ability to advance your skills and knowledge.