Comment 12 for bug 1035221

For me, this was caused by my custom .desktop file. I downloaded Firefox, unpacked it to my home and created a .desktop file for it in ~/.local/share/applicaitons as so:

    #!/usr/bin/env xdg-open
    [Desktop Entry]
    Version=1.0
    Type=Application
    Terminal=false
    Icon[en_US]=firefox
    Name[en_US]=Firefox latest
    Exec=/home/local/ANT/naddeo/firefox/firefox
    Comment[en_US]=Firefox directly from tgz
    Name=Firefox latest
    Comment=Firefox directly from tgz
    Icon=firefox

The issue was caused by my Exec line. I had to update it to this:

    Exec=/home/local/ANT/naddeo/firefox/firefox %u

Then external link clicks starting working again. I found that out by looking at the .desktop file that the Firefox in the ubuntu repos uses at /usr/share/applications/firefox.desktop and copying that.