Waller,
First, you can downgrade to JFBConnect v5.1.2 by just installing that release in the Joomla install manager. There were no database table changes from v5.1 to v5.2, so they should work just fine if you downgrade.
Before you try to downgrade though, please try the following fix. In the /components/com_jfbconnect/controllers/login.php file, around line 138, you'll see:
$password = '$2y$' . $provider->secretKey;
Change that to:
$password = $provider->secretKey;
Honestly, I'm not sure why the password would be changed. The only change we made in JFBConnect v5.2.x regarding passwords was that one. The change was also due specifically for a bug in Joomla 3.2.0 that had to be worked around. Since you're on Joomla 2.5, that change isn't required and may fix you up.
If not, let us know. We'll gladly investigate further. Downgrading should be a quick fix though as well.
Thanks,
Alex