One of the new features that Firefox 8 introduced was the "Don't load tabs until selected" option. If yo have configured Firefox to automatically load your tabs from the last time you used it could take a lot of time to start if you had many tabs open. Here is how to speed it up.
Go to Firefox's Options > General and check the "Don't load tabs until selected" box.
From now on it will only load tabs as you click on them instead of loading all of them at once on startup. In Firefox 8 all tabs (normal and pinned/apps) will be loaded once you select them. However this is going to change in Firefox 9 (release date December 20th, 2011) where only regular tabs will not be loaded until selected, pinned/app tabs will load on start-up and prior to the last viewed tab.