Hopefully, though I doubt it will, since FF seems to be a Warzone variant, as opposed to its own thing.

: 2. Will we be able to play solo and offline?

Solo, maybe. 100% sure you won't be able to play offline, since the absolute only thing that works offline in Halo 5 is Solo Campaign. Even Theater requires an online connection.

: 3. Will there be other Firefight maps, since the trailer only shows a variant
: on Escape from ARC/Dispatch.

Since it's a Warzone variant, I'm sure all of the existing Warzone maps will be included, but they may be working on a map specifically designed with Firefight in mind.