Use sensible-browser instead of "mozilla".
--- fbreader-0.8.17.orig/fbreader/data/default/external.desktop.xml
+++ fbreader-0.8.17/fbreader/data/default/external.desktop.xml
@@ -3,7 +3,7 @@
 	<section name="Web Browser">
 		<program name="%browser" protocol="execute">
 			<action name="openLink" command="%command"/>
-			<option name="command" defaultValue="mozilla %1 &amp;"/>
+			<option name="command" defaultValue="sensible-browser %1 &amp;"/>
 		</program>
 	</section>
 </ExternalPrograms>