http://i.imgur.com/w404ozB.png
Xpra doesn't show favicons for some windows, instead it shows some kind of default icon.
This is Firefox on the screenshot above.
However, there's an important detail: when I start Firefox - its profile manager shows up (because I have multiple Firefox profiles), Xpra shows Firefox favicon in that window just fine, but after I choose a profile - profile manager's window gets closed and Firefox's main window shows up and it lacks its favicon and instead has that default one.
Oh, the server is on FreeBSD 11x64, and same happens for both FreeBSD 11x64 and Xubuntu 17.04 as clients.
As per wiki/ReportingBugs, please always specify the exact version installed, and search for existing tickets.
See ticket:1144#comment:9, you can apply r15828 by hand if you don't want to wait for the next stable update.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1563