The only good fights for FFC are galakras, protectors (if they're kept stacked up), spoils (if you open a lot of small boxes), and nazgrim if adds are stacked on boss.
It's likely a dps loss to use FFC on garrosh due to the low uptime on adds. The proc also only has a 5 yard range, so it will not hit adds on norushen very often.
When you look at the chart I posted with trinkets, you have to mentally "guess" the percentage uptime of aoe vs single target.
Example being Garrosh. If I had to guess, I'm doing a single target rotation on Garrosh 90% of the time, and semi-aoeing 6-8 targets 10% of the time. For the sake of simplicity we'll say the fight is 12 minutes long.
It's around a 13k dps difference between thoks and FFC on single target, and with 90% uptime on my single target rotation that would be 648 seconds on garrosh * 13k = 8.42 million damage difference.
90k dps difference between trinkets on 8-target aoe, with a 8% uptime. 10% = 72~ seconds * 90k = 6.48 million damage difference.
So it's slightly better to go with normal trinkets rather than FFC.