- Mark one tab as an App tab in Firefox 4.0 or 4.0.1 with TMP 0.3.8.6pre.110429a installed. (I also have Session Manager 0.7.5 if that proved relevant.)
- Add new tabs to the right such that the leftmost non-app tab is forced off-screen.
- Set up TMP to show scroll buttons on either end of the tab bar when needed.
- Now close the right most tabs until those buttons aren't needed anymore.
Are all tabs visible? In my tests, at least one will be missing and the ONLY ways to get it back involve to switching tabs groups or restarting.