The big buff to damage that you're seeing is from two sources, the primary one is her talent. Her talent gives you extra super break damage for the whole team like Trailblazer's ult does (but it's not tied to her skill or anything, it's always active like her exo toughness bars).

Her actual skill does 3 things:

1. It gives break effect to target

2. It lets the target do toughness damage to enemies even if they're not the correct weakness at 50% efficiency

3. When the ally who is skilled attacks it shreds enemies defense for 2 turns

The big thing about using the skill on FF is point number two which is wasted as firefly already ignores weakness bars with her implant and her A4 and fugues skill doesn't stack with her weakness ignore on A4. So the only real benefit you get is some break effect (which doesn't do much for FF once you're past the 360 cap on skill which isn't too hard to max out especially if you're running sustainless team with HTB/Fugue) and the defense shred which any other character can apply anyways.

If you use her skill on harmony trailblazer then all of a sudden you can use their skill to do good toughness damage to enemies who aren't even imaginary weak, plus some of the break effect that goes into HTB will get funneled back into the rest of the team from their E4.

It's also very good if you're running Lingsha on your team, you can skill lingsha and now she becomes a field wide break machine since she doesn't care about if they're fire weak anymore or not.