Firework Blade is a synergy ability in Final Fantasy VII Rebirth, used by Cloud Strife with Aerith Gainsborough as a synergy partner. It costs three synergy charges each, deals magic damage to a foe, and boosts their limit level by 1, giving them new possible limit breaks. This can be boosted up to a limit level of 3 once this is unlocked from their folio.
In the ability's animation, Cloud tosses his sword to the air, which Aerith empowers. Then, swinging Cloud's sword and Aerith's staff, they fire magic blasts at an opponent.
Obtained[]
Firework Blade is unlocked on Cloud's folio, and costs 5 SP. It is available at party level 1.
Use[]
Firework Blade costs three synergy charges each on first use, and four on subsequent uses, and will boost Cloud and Aerith's limit level by 1 until they are KO'd in battle. At limit level 2, Cloud's limit break is Ascension, while Aerith's is Planet's Protection. At level 3, Cloud's limit break is Finishing Touch, while Aerith's is Rising Fury. Aerith benefits greatly from being able to access her higher-level limit breaks, while Cloud also benefits albeit to less of an extent as his default limit break is already strong, making the ability useful for them.
Both Cloud and Aerith also have other partner combinations to boost their limit level. Cloud can use Partners in Pain with Barret Wallace, Savage Assault with Red XIII, Maelstrom Strike with Yuffie Kisaragi, or SOLDIER Moogle Class with Cait Sith. Aerith can use Divine Punishment with Tifa Lockhart or Planetary Roar with Red XIII.
Increasing Cloud's limit level simply gives him stronger attacks. In lengthy battles, the extra damage from these abilities can be beneficial, especially if a foe is staggered or otherwise vulnerable. As Cloud's limit levels all deal physical damage, their usefulness will depend on how the player builds him; they are strong even if Cloud is built toward durability or magic damage but not a priority to boost his limit level, while if built toward maximum 
Attack Power, it is ideal to level them as soon as possible. He can get 
Berserk from performing the full Punisher Mode attack string, perfect to do just before using a limit break to make it 30% (normal Punisher mode) or 40% (Prime Mode) stronger. If Cloud can already hit the damage cap with his lower level limit break, the higher level ones will not offer any benefit; in this case, Cloud would be a great candidate for wearing the 
Genji Gloves.
Aerith's limit breaks, unlike other party members', are strategic supportive abilities. Planet's Protection renders the party invulnerable against physical damage, which both protects them from a major source of damage (and the primary source of damage from many enemies), but also from being interrupted by physical attacks. Rising Fury will fill the limit gauge of every other party member, allowing them to perform their various attacks. In longer battles, either of these can turn the tide. As such, it is a high priority to raise her limit level, even if raising Cloud's is by comparison a bonus.
If Expeditionary Medal is equipped on either party member, they will start at limit level 2, meaning Firework Blade can only boost their level up to 3. If Enhanced Expeditionary Medal is equipped, they will start at limit level 3, meaning Firework Blade will have no effect on them; as such, the ability should only be used to boost the other's limit level, or for the damage output. Other accessories (namely Silver Brawler's Gloves, Golden Brawler's Gloves, Draconic Ring, Enhanced Draconic Ring, Limit Booster, Transference Module, Gi Warrior's Charm, and Gotterdammerung) and 
Limit Support Materia can also raise the rate limit gauges increase in battle, allowing them to use their limit breaks more regularly and get more from boosting the limit level.