Can I react with a hidden temporal breach to a raging firebrand when you play me trigger to get his discount twice?

Partly — you can react to the trigger and the replayed Firebrand's own "when you play me" does fire a second time, but you don't automatically get two separate 5-less spells out of it.

Timing works: the trigger is a chain item, so the state is Closed and your hidden Temporal Breach has [Reaction] (it must have been hidden on a previous turn, and Firebrand has to be at the battlefield where you hid it — a hidden Breach can't reach a unit at Base, CR 811.1.d.2). Breach resolves, banishes Firebrand, and its owner replays it as a new object, so a second "next spell you play costs 5 energy less" trigger goes on the chain. Playing the Breach itself costs [0] from hidden and the replayed unit isn't a spell, so neither consumes a discount.

The catch is the two discounts: whichever trigger resolves first applies to the next spell you play at that moment. If you let both resolve before playing anything, they stack onto the same single spell (10 energy off one card, floored at 0), not 5 off two cards — FAQ #12624. To split them you have to play a Reaction-speed spell in the window after the replayed Firebrand's trigger resolves but while the original trigger is still on the chain; that card takes one 5-off, then the second trigger gives the next spell 5 off.