First time replying to something here with a quote, so sorry if I screw up the format here.
Agreeing with Garland and Zidane's numbers, but Exdeath can just use Zidane assist to punish Straightarrow, when the stage isn't so big that Firion cannot interrupt Maelstrom. Firion can try to get up close, but a smart Exdeath is going to be doing things like throwing out Black Hole or just teleporting away. The long game favors Exdeath, so playing a patient runaway style is perfectly fine.
Lightning doesn't need to camp Firion though...he has literally no good answer to Thunder against a Lightning with reasonable reaction times and good spacing. She can sit in front of his face at Thunder range and everything he can do is disadvantageous compared to what Lightning's options are. Within Thunder range, everything is interrupted except Swordslash (which Thunder outranges) and Shield Bash (which triggers the melee response), just by continuously using Thunder. Firion's only options become to block and try to get into Swordslash range (which allows Lightning to mix it up with her HP moves, punishing blocks) or run away. Lightning easily camps above Firion's head and can punish almost all of his attacks on reaction with Thunderfall. She has an easy time getting in, an easy time punishing, and has an overall much better risk:reward ratio than Firion does. It's clearly favorable to her. 6-4 Lightning favor at the very least.
Firion-Prishe should also generally be 6-4 since, while Prishe has good speed, her range is such that every time she gets close but doesn't land an attack is a Lord of Arms or Straightarrow mixup, which favors Firion, and she has to contend with Emperor assist's Thunder Crest for many stages, which makes Firion impossible to approach while it's up and can be used to interrupt some of her attacks when she lands them. The risk:reward situation favors Firion, since Prishe is getting fewer openings to land hits and is getting less off of them than Firion is getting off of his hits. Her BRV damage, while excellent, is comparable to Firion's own (and he has superior DEF to Prishe), and Firion is having a much easier time landing HP hits compared to Prishe, with superior HP moves and more opportunities to land them. Firion holds too many advantages with his tools for the MU to be even.