Always pass -new-instance when launching Firefox browser (#25251)

@brendandahl suggested it would have a positive impact on launching
Firefox. Not sure that I saw any behavioral difference, though maybe
this is one of those "can't hurt" things. Wdyt?

Author: juj

test/common.py

@@ -116,7 +116,7 @@ def configure(data_dir):
 
 class FirefoxConfig:
   data_dir_flag = '-profile '
-  default_flags = ()
+  default_flags = ('-new-instance',)
   headless_flags = '-headless'
   executable_name = exe_suffix('firefox')