That second page is really incredible slow It almost kills Firefox, I would suggest you to review the libraries that you are using and what they are doing, your users won't be happy with that performance.
With regards to the problem itself: check what happens when you hide the div with normal javascript, the problem might be that Firefox isn't loading the contents of iframes that are hidden, so you would need to use a workaround like creating it off the screen. Anyway, the problem isn't that GetHTML fails, but that you aren't getting a reference to an FCKEditor instance when you try to read it.
Re: FCKeditorAPI GetHTML not working in FF
Re: FCKeditorAPI GetHTML not working in FF
Re: FCKeditorAPI GetHTML not working in FF
That second page is really incredible slow
It almost kills Firefox, I would suggest you to review the libraries that you are using and what they are doing, your users won't be happy with that performance.
With regards to the problem itself: check what happens when you hide the div with normal javascript, the problem might be that Firefox isn't loading the contents of iframes that are hidden, so you would need to use a workaround like creating it off the screen. Anyway, the problem isn't that GetHTML fails, but that you aren't getting a reference to an FCKEditor instance when you try to read it.