I'm an old school player who played from unlimited to masques and then took a break for about a decade. I always played that you could activate {firebreathing} after blockers were assigned. Is this still the way it works? I've started playing online and it looks like I need to do it after I assign attackers but before blockers are assigned. If that is true, {firebreathing} sucks now.
You can activate it whenever. Its instant speed so yes after blockers is ok
Hummm...I must be screwing something up online then...Hummm...
You might need to change settings online so that you can hold priority after blockers.
When you are in a duel online, look on the bottom left where all the phases are. When you right-click on the different phase icons you can click "stop on my turn" or "stop on opponent's turn" or "stop on both". This will have you hold priority after the opponent has passed priority on his turns for that phase.