Thanks to everyone for your comments. The problem appears to be caused by this entry in /usr/share/xfce4/helpers/firefox.desktop:
X-XFCE-Commands=%B -remote "openURL(about:blank,new-window)";%B; Copying that file to $HOME/.local/share/xfce4/helpers/firefox.desktop and changing that line to X-XFCE-Commands=%B; makes the problem go away. Bug #779330 looks relevant. For those who are curious, here are some additional notes on how I tracked it down: Running $ strace -f -o ~/Desktop/exo-open.trace exo-open --launch WebBrowser showed a reference to the firefox.desktop file mentioned above. This line seemed suspicious: X-XFCE-Commands=%B -remote "openURL(about:blank,new-window)";%B; And doing firefox -remote "openURL(about:blank,new-window)" from the shell would fail. A quick web search failed to tell me what "%B" does, so I changed the line to X-XFCE-Commands=%B;%B; That made the problem go away. The strace output also showed that the firefox -remote "openURL(about:blank,new-window)" process was failing (exit(1)), and then a second attempt was made to start firefox, but without the '-remote "openURL(about:blank,new-window)"'. That one succeeds. So I guess the failure of the first process is what triggers the crash recovery behavior. It then occurred to me to check my Stretch system, and it did not have the problem. Its firefox.desktop has X-XFCE-Commands=%B; so that's what I'm now using. cheers, mike
