[firefox] Use firefox 60 by default to use DataTransfer approach for file...

[firefox] Use firefox 60 by default to use DataTransfer approach for file uploading functional tests.

component/firefox/buildout.cfg

@@ -69,7 +69,7 @@ part = ${firefox-51:location}
 # installed in ${buildout:bin-directory}.
 # Installing ${firefox:} is not enough, because wrapper setting $LD_LIBRARY_PATH
 # would not be created.
-<= firefox-52
+<= firefox-60
 
 [firefox-60]
 <= firefox-download