Hello,
In this situation (with 2 or more targets), you need to keep in a corner of your head how many crits you have in store. I suppose you know how LB works to stack hot streaks (it was well explained in a topic a few weeks ago), so in this situation you need to check if after first LB explosion you get double crit or not. So you have to wait before taking your decision depending of outcome. What to do while waiting ? Casting a fireball, and being ready to cancel it if needed.
Here are outcomes of first LB explosion, I assume you are at 35+y so that you can use a gcd before your fireball hit target :
No crit on main target : you can't get 2 HS from the 2 LB regardless of your crit previously in store and the outcome of 2nd LB, so you can keep casting your fireball.
Crit on both targets : 1 : if you had one crit already => you get one crit in store+HS => if 2nd LB crit on main target you lose HS if you don't cancel fireball to use current HS. So in this situation you can totally consider canceling. 2 : If you have no crit in store you need crit on the 2 target of the 2nd explosion to overright your current HS, so it's more unlikely that you overright your HS tahn in case number 1. In this situation you can cancel fireball if more than 2 targets are getting hit by second LB, otherwise it's risky to cancel it.
Crit on main target and not on 2nd target : 1 : if you have one crit in store you get HS and 0 crit in store, so you are exactly back to situation 2 that I described above. 2 : If you had no crit in store you are ****ed : the failed crit on 2nd target canceled your crit earned on first target, so you keep casting fireball.
Those are just examples of outcomes, everything is more complicated when you have a fireball flying between the 2 LB for example, which is often the case. So I won't enter further into details, I think you got the idea. A good firemage have to improvise, keeping in mind that cancelling a fireball cost more if it's almost finish, and less if you just started it. You have to make the decision based on possible outcomes, knowing your crit chance, and number of target hit.
For other spells I don't think it's worth it. Flamestrike you just said why, fireblast and scortch are instants so you don't have the benefit you get from fireball of being able to use a gcd before a new crit is stored. Furtheremore you should play out of range of those spells if possible.
Hope this help, any if you guys can pm me in game I like to talk theorycrafting,
Jinkho, main mage at MEMORIES (lord, horde).