Since we now group by default:
It doesn't typically get in the way when minimized:
Of course that was when we used to accidentially duplicate the last few emails of the page, on the first of another.
However, when we no longer did that as bad - once you expanded a bunch you couldn't rejoin or hide, (plus we had removed unpin/unsticky).
This was all discussed in
Bug 30391 - "pin" option removed from chat
The new 6.0 IM bar (instead of IM tab) doesn't get in the way of emails:
But currently doesn't let you open more than one at once/move them around the page:
Bug 35367 - im bar in GNR needs popout