Keef,
It's a problem I've raised with IB, and it seems to be confined to FF with NoScript, and to do with what is cached / not cached by the browser. IB aren't doing anything about it, as they say it's a NoScript problem.
It's not bothered me enough to research it further, other than temporarily disabling NoScript and clearing the browser cache, which cures the problem.
If I find out what in NoScript is causing the problem, I'll let you know - but don't hold your breath!
SD