Noticed this go by in my debug-build-spew today: > [5463] WARNING: the property first_time_experience_button_label already exists > : file ../../../mozilla/xpcom/ds/nsPersistentProperties.cpp, line 532 Note the awkward newline before the ":". That's there because the NS_WARNING string has an explicit \n character, but it does not need one. (NS_WARNING adds the filename, line number, and a newline character after the provided string). This dates back to bug 153754 when this warning was migrated from printf to NS_WARNING (and the "\n" from the printf expression was accidentally left in). Filing this bug on dropping the \n so that the NS_WARNING displays on a single line without the awkward break.

(Side note: bug 1233189 is already filed on the issue that's causing this warning to be spammed -- on first_time_experience_button_label being duplicated in a .properties file)