Add mac support to the FF50 plugin and package a candidate xpi.

Review at http://gwt-code-reviews.appspot.com/1467809

Review by: codefu@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10402 8db76d5a-ed1c-0410-87a9-c151d255dfc7
diff --git a/plugins/xpcom/Makefile b/plugins/xpcom/Makefile
index 4ee7372..3fb1587 100644
--- a/plugins/xpcom/Makefile
+++ b/plugins/xpcom/Makefile
@@ -200,6 +200,7 @@
 	$(MAKE) lib BROWSER=ff35
 	$(MAKE) lib BROWSER=ff36
 	$(MAKE) lib BROWSER=ff40
+	$(MAKE) lib BROWSER=ff50
 
 SRCS =	\
 		ExternalWrapper.cpp \