This is caused by the removal of the -remote
command-line switch in Firefox 36. If you have Mozilla Firefox selected as your preferred browser in the Preferred Applications settings, the terminal (or, rather, exo-open
) is still trying to use that obsolete switch, which confuses Firefox.
See this bug report (bug #1429284) for more details.
In any case, there's a very simple work-around:
Open the Preferred Applications setting (found under Settings in the Xfce menu, or by running exo-preferred-applications
from the command line).
Click the Web Browser selection menu, and select "Other..."
Simply type firefox
(or, equivalently, firefox "%s"
), in all lower case, into the dialog that opens:

You should now have "firefox" (as opposed to "Mozilla Firefox") selected as your preferred browser. Now just close the Preferred Applications settings, and try opening a link from your terminal — it should work normally again.