If you’re wondering, like me, why Firefox is reloading all of your App Tabs (even when you have explicitly selected the option “Don’t load tabs until selected”)? Well, the reason is a design/usability choice made by the developers. You can find the full discussion in this bug comments.
Luckily, they will add another option to change this behavior. This has been suggested on this bug report. Feel free to subscribe there to be notified when this has been solved. In the meantime, be patient while all of your apps load (evernote, facebook, grooveshark, feedly).