component/firefox: use 78 by default

This affects seleniumrunner and testnodes

component/firefox/buildout.cfg

@@ -92,7 +92,7 @@ cache-dir =
 # installed in ${buildout:bin-directory}.
 # Installing ${firefox:} is not enough, because wrapper setting $LD_LIBRARY_PATH
 # would not be created.
-<= firefox-68
+<= firefox-78
 
 [firefox-78]
 <= firefox-download