One thing that not nearly enough foxes do against Jigglypuff is effectively timing the wavedash forward out of shield both to escape the corner and to counter Jigglypuff's bair wall. Mango does it really well in a bunch of matches against hbox (fox vs. puff) but I'm at work right now so I can't really go video hunting. Wavedash forward oos jab is good. You have to do some reading as to how the Jigglypuff is going to place his aerials - rising bairs, falling bairs. like, if the jigglypuff is going to fade away and land after the next bair, you can wavedash forward oos and jab/dair/nair into stuff. If the jigglypuff is going to do a rising bair to pre-empt your jump oos (which brings me to an important point, by the way - DON'T jump around too much! a lot of foxes try to cross "over" the jigglypuff to escape the corner, and you can get ****ed over for this - watch m2k vs hbox on rainbow cruise) then you want to cross under the jigglypuff and possibly do a retreating bair as you reclaim space, hitting the jigglypuff from behind.
Soft nair/bair combos into usmash. Normal nairs at higher percents (~90%) combo into uair. So you've got ways to kill besides uthrow usmash. Although, just being patient and trying to find an opening to grab is pretty effective too.
When you're in close quarters, hold shield and a direction to pre-empt the uthrow rest. Don't get stupid with your tech patterns on platforms. Also, don't dashdance into jigglypuff's pound.
I think it was Lucky who said that on platform stages, it's good to camp the platforms to make jigglypuff approach you, and since Jigglypuff has such poor VERTICAL aerial mobility (unlike her superior horizontal aerial mobility) you then, as Fox, retreat to the ground, get under Jigglypuff, and sneak in bairs behind her. Which makes sense, although I've never had a chance to try this out because A) I live in Japan where there are no good jigglypuffs and B) we only play FD over here. D:
Oh yeah, I personally really like FD vs. Jigglypuff but hey, it might just be because I've never fought like a hbox-level puff.
Lastly, I think there's a lot to be said for honestly just running away and lasering until Puff is at 60%, and then never approaching and purely seeking grabs (for uthrow usmash) or guaranteed combos into usmash (soft nair over the puff, for example). Most examples of camping failing vs. Puff are when the Fox gets impatient and finally does decide to fight, I think.
I hope I'm qualified to give advice on that matchup lol. It's all theorycraft though, cuz again I've never fought a really really good puff. Somebody be sure to point out anything I said wrong.