Firefox 9 is going to bring a change to the new Don’t load tabs until selected feature which was introduced in Firefox 8. In Firefox 8, when Don’t load tabs until selected option is checked, ALL tabs (normal and pinned/app) don’t load until selected. However, in Firefox 9.0 (release date December 20th, 2011) only regular tabs will not be loaded until selected, pinned/app tabs will load on start-up and prior to the last viewed tab.
There was talk in Bug 674452 of adding a Always restore pinned tabs sub-option (checked by default) under the Don’t load tabs until selected option, but that was shot down. The main reason I use app tabs is they take up about 85% less space than a normal tab. This change should prove to be rather interesting and confusing for users when Firefox 9.0 is released.