Can not paste primary X selection in Firefox any more

[Martin Husemann <martin%duskware.de@localhost>]

Hey folks,

I just updated lots of pkgs on this machine, including most of Xfce4
and Firefox - so I don't really know what to blame, but I have a serious
regression:

I can not paste the primary X selection (the clipboard copy + paste works
fine) into Firefox any more.

I rely on that a lot, by having an xterm with ssh to some other machine
and a program there generating data that I need to copy over into a web
form or similar.

This always used to just work by pressing the middle mouse button over
an edit field in Firefox - but now it doesn't any more.

Simple test case: open a terminal, say "echo wwww.netbsd.org", mark
the output line, switch over to firefox and middle-click on the URL bar.

Does this work for anyone with firefox-59.0.1? It might be the xfce4 window
manager or some obscure security setting or whatever.

Thanks,

Martin