Firefox 4 introduced app tabs, a highly convenient way to organize your favored Web pages. Instead of normal, wide tabs that open at the top of the browser, an app tab is a tiny tab that sits to the left. App tabs are convenient not only because they take up less space than a regular tab (an app tab is hardly any wider than the favicon it displays), but they also cannot be accidentally closed. When you quit Firefox and reopen it, however, you'll soon discover that your app tabs aren't saved from one session to another.
If you have only a handful of app tabs, it's easy to simply pin pages as app tabs each time you open them up. But if you use more than a handful of app tabs, the pinning process session after Firefox session is annoyingly time consuming. There is a word-around to this problem, though it's not perfect.
To save your app tabs, go to Firefox Options (Preferences on Macs) and on the General tab, use the pull-down menu next to the "When Firefox starts" header to select "Show my windows and tabs from last time." The problem with this solution is that all tabs from your previous session are restored--not just the app tabs. Also, Mac users: if you close Firefox by clicking the red X, your tabs and app tabs will not be restored. You must quit out of the app (command-Q) in order for the app tabs to reappear the next time you start Firefox.
For more on app tabs, see Download.com editor Seth Rosenblatt's video here.