Fox:
Adv.
Can camp amazingly with lasers
No trouble KOing Kirby since he's very light
Much faster ground speed.
Ftilt can combo into running U-smash at about 70-80%(stale)
Disadv.
Can be KO'd at about 90 by Kirby F-smash
Can be chaingrabbed to about 40-50%
Has trouble dealing with Bair camping imo
Inhale offstage has a guarenteed footstool on Fox making it very hard to get back(probably guarentee KO)
If Kirby Swallows Fox, he can SHQL
Kirby has a better edge-guard ability than Fox
Kirby can also crouch under lasers so making them hit will be a little hard.
Stages: Fox would most likely strike YI,and PS1, Kirby would strike FD and BF. Pretty sure it would end up at SV
Fox should ban Frigate, Kirby should ban FD.
Go BF or Halberd if FD gets banned, Kirby should go Delfine for a CP.
Summary: Since Kirby will most likely start off the match crouching, and Fox would start off laser camping, nobody will move unless someone makes the first move.
With Fox, I personally run towards Kirby and pop out my sheild and pressure the Kirby to attack or go for a grab(dont get within grab range), when kirby trys to attack, I run back and start camping again, and if they start crouching I repeat until the Kirby player starts to attack me or pressure.
In the air: Kirby should have an advantage here thanks to his Bair train. Punish the bair by waiting until the hit frame is over and go in for a auto-cancel Fair, or just laser camp if the Kirby is bair camping. If he's really close to you, try powersheilding his Bair and go in for a running U-smash.
Ground game: I feel like Fox has an advantage here because of his Dair follow-ups, you may get about 2-3 Utilt's before Kirby can get out. Kirby should try getting the grab for some easy percentage. But getting the grab may be a problem due to Fox's amazing walking and running ability. Fox's F-tilt gets outranged by Kirby's F-tilt. Kirby's D-tilt is great because it can trip Fox even at high percents and combo into a F-smash. Kirby u-tilt can lock Fox for about 30% I believe so look out for that.
Sorry guys this is all I can write atm, I have work:/ But if someone can critic my advice and see if there is anything wrong with what I said this will also help. Peace.
Edit:
Approach: I personally never approach against Kirby. I just laser camp SHTL all day until the Kirby gets close. When Kirby goes for grabs and misses, punish the grabs, Kirby running grab has a lot of cooldown and it's easy to see coming.
With Kirby I would crouch walk up to Fox but not within his range(crouch walk is when you hold down then let go of down and move a little to the right/left then crouch again) All I can say is D-tilt him and Bair for approach.
Camping: Fox camp with lots of lasers, Kirby B-air camp. I don't think there is a lot of camping in this MU. I guess Kirby fair can be used and retreated?
Edge game: Fox camp with lasers and recover with Side-B. Kirby should B-air the Fox and use D-air if Fox uses Up-B.
Surviving: An U-smash from Fox should be DI'd up and away to the right( if Fox U-smashes faces the right) and DI up and away to the left(if Fox U-smashes facing the left). Kirby's F-smash should be DI'd to the corner with U-air. Sorry this is all I have.
Killing, Fox's U-smash kills at about 90% if fresh with DI, Kirby F-smash kills at about 90% if fresh. Fox's D-smash kills at around 110% depending on where on you are on the stage, if you're near the edge, it kills at about 90%. Kirby U-smash kills Fox at about 100%. His D-smash kills at about 105% I beileve. I don't think B-air will kill because Kirby will be using that a lot in the match, but fresh it kills at about 110% depending on what part of the stage you are. Fox U-air kills at about 110 or 120%, his B-air kills at about 120 or 130%.
Well these are my 2 cents. Please correct what is wrong if you see stuff.
I think 55-45 Kirby or maybe 50-50