Removing usage of the obsolete xulrunner binary

--- usr/bin/pencil
+++ usr/bin/pencil
@@ -1,5 +1,15 @@
 #!/bin/sh
 #
 
-/usr/bin/xulrunner --app "/usr/share/pencil/application.ini"
+DIR=/usr/share/pencil/
 
+if which firefox 2>/dev/null ; then
+    firefox -app $DIR/application.ini "$@";
+elif which iceweasel 2>/dev/null ; then
+    iceweasel -app $DIR/application.ini "$@";
+elif which abrowser 2>/dev/null ; then
+    abrowser -app $DIR/application.ini "$@";
+else
+    echo "Could not find XULRunner runtime engine."
+    exit 1;
+fi