This addon removes the checks google put in place to disable Firefox accessing Google Inbox, the new mail webapp. By installing this addon these checks will get removed, so accessing https://inbox.google.com should just work. (No need for diving into the preferences. There are none). From personal limited testing there is nothing that break. I have used Google Inbox without issues with this plugin.
If there are issues, please let me know. E.g. by opening issues on https://github.com/h4writer/enablegoogleinbox/issues
Note: this addon is opensource so feel free to view and improve code: https://github.com/h4writer/enablegoogleinbox