As of Patch 3.0.2, Firestones are Warlock-only temporary weapon enchants akin to Wizard oils. A Firestone increases all spell direct damage done by 1% and increases Spell critical strike by 7/14/21/28/35/42/49. They are Bind-On-Pickup, but unlike other Conjured Items, Firestones and Spellstones do not vanish after logged out for any amount of time (Much like a Wizard Oil created by Enchanting).
Original Firestones
The original Firestone was an off-hand slot conjured item which was later moved to the wand slot (technically a relic), created by the warlock 
[Create Firestone] spell, allowing warlocks to enhance their main hand weapon to do additional fire damage and gain a fire spell damage bonus buff for as long as the firestone exists. When active, a Firestone,stacked with weapon enchantments, and placed a small fireball that circled the Warlock's weapon.
Both the firestone and spellstone were widely unused for multiple reasons. They did not provide comparable stats to Wands and, like other conjured items, would disappear if logged out for more than 15 minutes. This was changed in Echoes of Doom, Patch 3.0.2, to the current state. The change also included the Firestone's companion, the 
[Spellstone].
Ranks
| Types of firestones | |||
|---|---|---|---|
| Type | Item level | Damage dealt by direct spells | Spell critical strike rating |
| [Firestone] | 28 | +1% | +7 |
| [Firestone] | 36 | +1% | +14 |
| [Firestone] | 46 | +1% | +21 |
| [Firestone] | 56 | +1% | +28 |
| [Firestone] | 66 | +1% | +35 |
| [Firestone] | 74 | +1% | +42 |
| [Firestone] | 80 | +1% | +49 |
Tactics
As of Patch 3.0.2, Firestones and Spellstones are the preferred temporary weapon enchants for Warlocks. Firestones are preferred for Warlocks that favor direct damage spells (Destruction and Demonology warlocks), while Spellstones are preferred for Affliction Warlocks, who favor damage over time spells.
External links
Wowhead Firestone info.
Thottbot Firestone info.
Allakhazam Firestone info.