Ive never done one on a fircat, but I did on a ZL. I just loosened the starter so you could move it back enough to remove the shaft. The shaft slides onto the starter and the gear. They have little allan lock screws on each end. The was the hard part, because they rusted and were a bitch to get loose. Hopefully yours isn't so bad. Otherwise it wasn't a bad job.
Finally figured something out. Only about an inch wide space between the engine and cooling at the front of the tunnel. Did loosen starter(don't think I had to after all) Got cable turned to where allen heads were pointing up that opening. Took 2 long 1/4" extensions and a socket set up with 3/32" allen head. That wasn't bad once I came up with that, the tough parts was trying to get the new one on without a straight shot at it. Bent the cable for a while to try and get a bit of a curve on it. Slid box end wrench over cable to help pull it up into place and eventually worked in onto the shaft so I could tighten it. Don't wanna do that again anytime soon.
Removed exhaust, driven clutch, trim around handlebars, & left side engine/chassis bracket.