I'm not sure, if there are any add-ons, but there are many other programmes (also freeware), which are not based on firefox, but able to open .mobi files. Use google search :)

You would probably need to install a program that offers a plugin for Firefox if you want to open a file type in Firefox that isn't supported natively.
Plugins add features like handling file types that aren't supported and extensions add new features to Firefox or modify existing ones.

• https://support.mozilla.org/kb/find-and-install-add-ons-add-features-to-firefox