So the issue with that particular plugin is that it tries to update a button element to set the text on. We have an input element as the Register button. The javascript that it's calling does not work on an input.
You can temporarily make a template override for components/com_jfbconnect/views/loginregister/tmpl/default.php
<input type="submit" class="btn validate <?php echo $this->configModel->get('registration_registerbutton_class', 'btn-secondary');?>" value="<?php echo Text::_('COM_JFBCONNECT_REGISTER') ?>" />
to
<button type="submit" class="btn validate <?php echo $this->configModel->get('registration_registerbutton_class', 'btn-secondary');?>"><?php echo Text::_('COM_JFBCONNECT_REGISTER') ?></button>
I don't think it would be a huge deal to change the element type in the next release. It's more consistent with com_users, but may break some styling.
-Melissa
[9.0.244 Bug] Social Registration page CAPTCHA
