2.5.2. error

That's an odd one. We've used the JFile class in the authentication plugin forever, and just added a similar check to the user plugin (to ensure that JFBConnect is installed properly before trying to use it).

If you could, in the /plugins/jfbconnectuser.php file, add the following line into the onLoginUser function at the top:
jimport('joomla.filesystem.file');
jimport('joomla.user.helper');

Sorry for the inconvenience. We tried to prevent a bug by inserting that check, and introduced another. My assumption is that on our site (and others), some other function is importing the filesystem library, but on yours, there isn't.. hence the not found error.

We use them because they're there. Generally, for good practice, we try to use the Joomla libraries where possible. In some cases (certainly not all), they wrap around standard PHP functionality and give enhanced compatibility across different types of servers/configurations (windows, linux, etc).

We just uploaded 2.5.3. That was the ONLY change, so don't feel like you need to get it if your running now. We're hoping to ride this one a little longer than 1 day. If there's no more issues, we can get back to feature improvements

Thanks a ton for all your feedback!