basically yeah you want to play defensive but you also want to punish his whiffs. kirby is a very aerial happy character, use that to your advantage. hes always going to approach you since you have lazers, keep it that way, remember your in control of the match. in order to punish his bair's you must identify/predict when he will do them and punish depending on spacing.if you dash shield them you can short hop auto cancel fair them. this will rack up allot of damage and provides many opportunities for follow ups.The mistake you made is thinking your aerials suck or fox sucks in the air, fox has alot of solid options in when fighting airborne. if they get to close either airdodge through them, full hop dair or nair or even roll, you dont have to block everything.
try not to get grabbed early on as it will give kirby a decent lead, however dont try and escape close quarters combat range all the time. most of the time you will want to punish there attempts with grabs and uptilts. use crossups to mix it up from time to time. kirby is small so it is allot easier to jump over kirby with a short hop than say snake.dash in grabs are always a good idea. when below kirby go for the uairs. its hard for kirby to fight back as dair is not that fast to start up.Remember to play patient, look out for holes in there game play, look for patterns an abuse your speed.
if you want more depth into the MU ask TKD nicely and see if he can give you some tips as he is probably the most experienced fox against kirbys (playing mogX allot and playing chudat in a tournament set). hopefully i helped