There is a problem when firefox have multiple opened pinned tabs with a small width :
-"Arrow" and "Plus" buttons overlaps.
--We can't view current selected tabs (i mean the tab header not the window content)
From my opinion and my workflow, it could be really usefull to handle them in another way :
- minimize opened pinned tabs with a simple vertical line(instead favicon) until there is not enought size.
- add a cursor to let user adjust this minimization.