um, I think the solution was a new version

I honestly do not remember... All I know is that the newest WB and FF3 works.
Make sure it is set to use spellerpages in fckconfig and only do the change in spellerpages.php if you are on a linux based system. Are you sure you are using spellerpages in IE and not iespell?
You also need to clear your cache after you make any changes to the config files.
I can offer you one more suggestion. However, I think this was only implemented in the recent fckeditor and you need to the newest WB to get it.
In fckconfig.js there are some more config lines that make it friendlier to FF.
FCKConfig.FirefoxSp
ellChecker and FCKConfig.BrowserCo
ntextMenuOnCtrl
The first one will let FF mark misspelled words with the redline underneath. The second one will let you bring up the FF context menu only while you are holding down Ctrl. This will allow you to fully utilize the FF spell checking feature by seeing misspelled words and then you can correct them through the CC context menu by holding down ctrl while right clicking.