Sorry that you've run into this issue. The new version of FireFTP is doing migration of an old account format to a new one. Unfortunately, there are some illegal characters that make it difficult to upgrade to the new format that Mozilla now requires. (specifically, the combination of trying to convert international characters and the '%' character creates problems).
If the latest version isn't working for you, you can try the following:
find your profile folder, here are instructions: http://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data (if you're on a Mac it'll be under: ~/Library/Application Support/Firefox/Profiles) find the file in your profile folder called fireFTPsites.dat remove the '%' characters found in the file. remove any characters that start with \x, remove any characters that might have umlauts (like ä) install the latest version and readd the % characters back in later as needed. (not directly into the file, but via the regular FireFTP interface). The same applies for the files you're trying to import. If you have further issues, email me http://nightlight.ws/mime/about/contact and I'll help further.
The Best! You could have the option to only activate it separately. Because sometimes I want to access the FTP through the browser and not through the extension. And it's hard to have to disable the extension, restart Firefox for this.