There's a fairly significant period of time before your notification of a firestarter proc registers, assuming you're talking about getting the Firestarter proc on the very last Fire, not from the second-to-last Fire. If that's a full half second, only 20 of those would add up to 10 seconds, which would be ~4-5 GCDs. If it's a longer delay, then that's even more GCDs that could be fit in there.