Fairly buggy on Firefox under Ubuntu Linux. It seems to work better on Windows 10, OSX (both Firefox) and Android though.
Main issue I have is the pop up when clicking on icon doesn't stay open. I have to right click and sign in from webpage, which is an ok workaround. From there, the pop up seems to stay open and work fine. As long as you're aware of this workaround you should be ok.
Earlier issues I've had with Ebay login seems to be fixed and working now.