only the newer versions >V2.+
I'm on Firefox/1.5.0.12 ( I know I'm a fossel but many of my extention won't work on the newer Vs. The newer Vs are so bloated. sadly some newer extentions aren't backward compatable. It's a trade off) I know of no spell check compatable with my version.
I was thinking mainly of the guys runing IE. Been so long since I used it for, all I know it could have a built in. Last time I used IE, spell check worked through Adobe Acrobat
it's been a while.
It was just a thought.