You need at least the first badge to use flash.
The 7th badge
In Pokémon FireRed, for defeating Brock, his Badge will allow you the use of the Flash HM, Misty's Badge will allow you to use the Cut HM, Lt. Surge's Badge will allow you the use of the Fly HM, Erika's Badge will allow you the use of the Strength HM and Koga's Badge will allow you the use of the Surf HM.
The 7th badge.
Soul Badge (Fuchsia Gym)
You need at least the first badge to use flash.
The 7th badge
In Pokémon FireRed, for defeating Brock, his Badge will allow you the use of the Flash HM, Misty's Badge will allow you to use the Cut HM, Lt. Surge's Badge will allow you the use of the Fly HM, Erika's Badge will allow you the use of the Strength HM and Koga's Badge will allow you the use of the Surf HM.
The first badge in the game. He also allows you to use HM Cut outside of battles.
The 7th badge.
Soul Badge (Fuchsia Gym)
You can use the move strength in Pokémon to move boulders. You can use strength in FireRed after getting the rainbow badge.
First you need the first badge then you need the HM Flash which is given by the oak aide on route 2 lastly go inside rock tunnel and choose the Pokemon that has flash then choose flash the cave will be lighted up more!
get the 4th badge in celadon city
The Thunder Badge
get it from this fisherman i think
Hm05 is flash..if you need flash you need to win the 1st gym badge. You need to beat brock