Sometimes I use that instead of the icon because the icon pops up with the thread in a new window
Only the Flash one uses a new window (since it would close Flash otherwise, and the Flash app takes time to load so it's preferable not to close it.) If you're not in the flash app, it will load in the current window.
and also that icon goes to the beginning of the thread, not the end like you get when using the mail tab.
Good catch, I'll fix that.