Cookie Importer has now been enabled for Firefox 4.0.*. If you experience any problems in that combination you can use the Compatibility Report tool from Mozilla or write me a mail.
I'm currently hunting for a specific bug, that raises an exception like this:
"Component returned failure code: 0x80070057 (NS_ERROR_ILLEGAL_VALUE) "
If you got such a message then, PLEASE write me a mail and append the cookies.txt that you wanted to load (if you're paranoid check the contents and remove any obvious mail, banking, etc. cookies from the file. Check then if the exception still raises).