Fiora does not scale with crit at all. it is a horribly inneficient stat on her.
Q does not crit.
The first hit of E NEVER crits.
The second hit of E ALWAYS crits.
W is not capable of criting.
Your ultimate does not scale with crit (unless you count them dying before you finish the ult, which actually results in LESS of a heal).
Fiora is an ad caster who applies her abilities through auto attacks, not a melee adc like she used to be (hell, even then she still built raw ad and ulted people to death instead of a crit build).
Your basic rotation generally consists of q-e1-e2 w to stall for time or block a cc - q again. At no point in there is there even time or room to go for more than maybe 1 or two auto attacks that are even capable of criting. This is even more exaggerated during the ultimate, where you spend any free time between q's outmaneuvering to try and hit all four sides once. There's no time to stand around doing basic autos when you have q, 2 es, and moving around to consider.
The mana is nice, I'll give you that. But it doesn't make up for the horrible gold sink investing anything into crit is on fiora. If it's not super early game and you've getting lots of autos off on someone that aren't part of your e, you're probably fucking up royally somewhere.