Firefly, I watched the Fox matches against Bundtcake because I know the Fox matchup VERY VERY well, so I can offer some advice. Whenever you fight a Fox, you HAVE to keep the pressure on (similar to when you fight Samus). If you bide your time and camp, then the lasers will just slowly rack up damage. Eggs from long range are virtually useless in that matchup because of the reflector, and you should only use them when Fox is in the air or trying to recover.
On your first match the first life, he was DESPERATELY trying to kill you with that up-smash and you shouldn't let him. Fox is great at racking up damage, but when it comes to kill moves, his only reliable ones (without some set up) are up-smash and back-aerial. If you constantly watch out for those when you're around 95% and up (you can live a fully fresh up-smash at 110% if you DI it properly, but to be safe look out for it earlier). The other kill moves are his down-smash, up-aerial, and forward-aerial. The down-smash (like the up-smash) can be set up by a down-aerial.
I know I said earlier to keep the pressure on, but when you're at a high kill percentage yourself, get away so that you can rack up as much damage as you can because a few more laser hits doesn't matter at that point. The up-aerial is easy to see so that shouldn't be an issue. If you get caught in a forward aerial, smash DI down as if your life depended on it. If you do, then you can get out of the last hit (which is the one that has all the knock back and will kill you), and that also puts you in a great position to counter the ending lag of his move with an up-aerial.
If you get caught in an up-tilt spam at low percentage, jump out of it. You can get out on the second hit with the super armor and not take 50ish damage. If you grab a Fox player near the edge (see the thread about chaingrabbing Meta Knight and where you should be for the spike after the release grab is over) and you're near that magic spot, you can get a good gimp. If the Fox doesn't jump immediately, you get a spike, and I'm not sure on the percentage, but it doesn't take much before you can edge guard him after the spike and get the kill. If you think Fox is going to jump instead, you can do a rising neutral aerial and that'll hit him out of his jump. After this, most Fox players will do a side-B to get back on the stage because they'll be high enough. Try to wait as long as you can to see whether he'll go for the edge or go for the stage and go for the edge-guard or counterattack. If you read it quickly enough, you can run away and get a forward smash off, and if not, you can always do a short-hop neutral aerial. This ALWAYS at least clinks with the move and sometimes beats it if you get it out fast enough.
If you grab a Fox at low percentage in the middle of the stage, try an up-throw. Most Fox players will use the reflector to stall in the air and then counterattack with a down-aerial since they're right above you. You can do a short-hop up-aerial to knock them out of the reflector if you predict it, or you can wait for the down-aerial and pivot grab it. Keep in mind that this is situational and I'm just saying what happens most times. It all depends on the player, but try it out regardless.
As far as approaching goes, Fox LOVES to attack from the air, so you have to counter from the air if you're going to approach. Space your back-aerials and egg-lay to rack up the damage. If you get the egg-lay, you have a guaranteed up-smash if you hit the egg, and if you space it correctly, you can't be punished for it (some might argue with me on this, but if they want to test it, be my guest). As far as your kill moves, it goes back to Fox's kill moves. He'll try up-smash and back-aerial first. If you play away at high percentages, then he'll fall into trying to get a down-aerial so that he can get the up-smash or down-smash after it. Try to space this and you can get a forward smash on him if you can read him well.
Mostly the match is that, but a lot of it boils down to reading your opponent. Fox is extremely fast, but unlike Sonic, he can actually do SERIOUS damage to you and kill you early if you don't know what you're doing. Try out what I said next time you face one and good luck.
