Odd... there's a report of the same thing at the Mozilla base...
http://support.mozilla.com/tiki-view_forum_thread.php?locale=nl&comments_parentId=491951&forumId=1
Doesn't list any solution or explanation.... FWIW, the location of that file is...
/Applications/Firefox.app/Contents/MacOS/firefox-bin
Assuming that you installed Firefox in the default Applications folder (if not, put the rest of the path in before Firefox.app). If Onyx allows you to put in a location for the firefox binary, try this one and see what happens.