There really is a Fire Lake in the Uper Peninsula of Michigan, Bob Seger's home state... but the song isn't really about an actual place. It's about a dream- a dream that inspires people to abandon their inhibitions and their boring old lives in search of something more exciting.
One theory that makes sense to me is that "Fire Lake" is actually Michigan's Silver Lake, which used to be a mecca for motorbikers. In the Sixties, a kid from an ordinary middle class family in Michigan would see motorcycle gangs heading off the Silver Lake, and would hear rumors about wild parties and scantily clad biker chicks... and it sounded EXCITING. That kid probably dreamed of how cool it would be to get his own motorcycle and head out on the open road to a place where he could do ANYTHING he wanted, and where he could find gorgeous, sexually uninhibited biker mamas.
Even if a middle class kid KNEW he was never likely to do anything like that in real life, the dream was enticing.
Who is "Uncle Joe"? Probably a typical, ordinary middle-class married man who got bored with his life and left his family in pursuit of the freedom and excitement he's never had. For him, "running off to Fire Lake" just means having a midlife crisis, chucking all his responsibilities, and going off to have fun.
In this song, Bob Seger is saying, "Whoever you are, whatever age you are, don't settle for a boring existence. Go out and pursue your passions."
Fire Lake represents all the wild, crazy things you've always wanted to do but never dared to try.
