From Sonic Retro
The Fire Shield or Flame Shield, known as the Flame Barrier (フレイムバリア) in Japan, is a power-up first introduced in Sonic the Hedgehog 3. It is a variant of the standard Shield which in addition to giving its holder an extra hit point, makes them impervious to fire or lava.
Sonic the Hedgehog 3 and Sonic & Knuckles
The fire shield first appeared in Sonic the Hedgehog 3 as the flame shield, reappearing in Sonic & Knuckles (and by extension, Sonic 3 & Knuckles). When used by Sonic the Hedgehog replaces his Insta-Shield double-jump move with a rapid horizontal fireball attack, allowing him to blast forwards at a high speed and break any objects that a spin dash can.
If taken underwater, the flame shield disappears, and like all Sonic 3 shields, it will repel enemy projectiles.
Sonic 3D: Flickies' Island
The shield returns in Sonic 3D as the red shield (or red barrier (レッドバリア) in Japan).
In Shadow the Hedgehog, the Fire Shield causes minor damage to any enemy that pierces it.
It returned in a few challenge acts in Sonic Generations with its dash attack and fire protection intact. When the "Flame Shield Challenge" has been completed, the Fire Shield can be bought in the skill shop and, if equipped, can be activated at any time for Classic Sonic by pressing /.
The Fire Shield appears in iOS/Android remake of the original game and its sequel if you set "Items" to "S1+S3" or "S2+S3" in the Level Select menu (or place a Fire Shield monitor in debug mode). It behaves exactly as it does in Sonic 3 & Knuckles, including the fireball dash and disappearing when it touches water. In the Sonic 2 remake, the Fire Shield also appears as a random item in VS. Mode.
It appears again in Sonic Lost World as a power-up the player can receive from Miiverse in the Wii U version, or found within levels in the 3DS version. It does not come with the ability to perform the fire dash, but it still protects against fire.
The Fire Shield appears again in Sonic Mania, where it can burn down bridges in Green Hill Zone and set the oil on fire in Oil Ocean Zone. It also protects the player from freezing in Press Garden Zone and destroys anything made of ice by contact.
Other variants of the basic shield include the Lightning Shield, Water Shield, and Gold Shield. An item similar to the Fire Shield appears in the 2010 game Sonic Colours, in the form of the red Wisp.
Fire Shield monitor (to Sonic's left) in Sonic & Knuckles
Active Fire Shield in Sonic the Hedgehog 3
Fire Shield used for Classic Sonic in Sonic Generations
Fire Shield in Desert Ruins in Sonic Lost World 3DS version
Sonic burns a bridge using a Fire Shield in Green Hill Zone Act 2 of Sonic Mania.