The Fire Shield or Flame Shield is a power-up first introduced in Sonic the Hedgehog 3. It is a variant of the standard Shield, and like most other items is found inside monitors during gameplay. In addition to serving as a one-hit barrier against damage / life loss, the Fire Shield makes its lucky possessor invulnerable to fire-based attacks, allowing them to stroll across boiling lava and take flamethrowers to the face with a complete absence of concern.
While all variants of this shield prevent fire damage, its initial incarnation in Sonic the Hedgehog 3 (and, by extension, Sonic & Knuckles and Sonic 3 & Knuckles) is lost if the character goes underwater; and when used by Sonic 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. Sonic 3D had a Fire Shield, but no water to lose it in and no double-jump attack for it. In Shadow the Hedgehog, the Fire Shield causes minor damage to any enemy that pierces it. It returned in a few missions in Sonic Generations with its dash attack and fire protection intact.
The Fire Shield appears in iOS/Android remake of the original game 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.
An item appears again in Sonic Generations unlockable as skill for Classic Sonic and functions as in Sonic 3 & Knuckles, to defend from the fire.
It appears again in Sonic Lost World with the same function as the previous games.
The Fire Shield appears again in Sonic Mania in where it can burn down bridges in the second act of Green Hill Zone. The Fire Shield also has the added ability in Sonic Mania to act as a homing attack, possibly a reference to the Gold Shield, when in close proximity of Badniks.