I personally have not been queuing my Fires since learning Firestarter as I don't want to waste the time cancelling. It doesn't seem beneficial to continue a Fire cast as it has the chance to proc another Firestarter and you missed out on the opportunity to get two back to back.

Also regarding Thunder, I haven't learned Thunder 3 yet, but I've been using Thunder 2 as an opener since the high 30s and on since mobs last a little bit longer now. Getting the chance for the Thunderspark proc seems worth it IMO. I am pretty sad they disallowed the stacking of the three Thunder spells. I used to use these as a filler waiting for my mana to recharge after swapping to Ice.