Wow, that Scott Guest is really an a$$hole. Don't let him get to you. I think based on what you said about your case, you've still got an excellent chance at becoming a firefighter in the State of IL. Good luck & stay safe!
It would depend on the felony.
For instance; if you were being hired to the banking industry, theft would be a bad one to have on your record.
The one that I see the most that causes applicants trouble are violence/weapons related.
Drugs. Unless you are convicted of manufacturing with intent to deliver or are caught with enough to be considered selling, I would think that you could get letters from people that you know that would aid your situation.
Oh and pick a better class of friends.