Even 5 years ago, it was told to me like an urban legend that "fire stuns".
I have done some testing with the noko ammy (the one that adds fire dmg) and found nothing to the effect of "stunning" an enemy. I have found that using knockback will put a "stun" like effect on an enemy that IS NOT effected by FHR. It has a set animation.
Hit recovery is also caused by taking more than 1/12th of the enemies life in a single blow (pretty sure of this). This is why Hammerdins lock enemies with their 5-8k actual damage hammers (8k-14k LCS).
Fireball has a built in stuneffect afaik. Fozen armor can freeze a target which is diffrent from slowing down an enemy.
The main reason why a monster falls into hitrecovery is the amount of damage been dealt.
Hmm, first I thought: NO way fire stuns...
But then I realised that on my fire sorc lot's of times I have been able to keep a single enemy from getting to me, just with quick firebolts, so now I think fire does kinda knock-back, but doesn't properly stun, like smite.