Yes Suckerfish is pure CSS - except in IE where some js is required in order for it to implement :hover.
My particular menu was kindly tested for me by users at another forum and worked on every platform-browser combination imaginable. This is not a bug as such, just a difference in the way FF3 renders the CSS.
Thanks NoDog, that is exactly what is happening.
I guess this will be the impetus for me to move over to Son of Suckerfish and iron out some of the IE hacks as well.