minor fix

* AppLauncher.uc.js: update error messages.
* userChrome.js: clearer define a no-operation function.
* userChrome.js: update comments.
rename |safeBlock| option to clearer |safeClose|
use default function parameter
fx34: use shorthand function property [MANY CHANGES]

* https://bugzilla.mozilla.org/show_bug.cgi?id=924672
fx34: use computed property name

* https://bugzilla.mozilla.org/show_bug.cgi?id=924688
fx34: use template strings [MANY CHANGES]

* https://bugzilla.mozilla.org/show_bug.cgi?id=1038259
minor fix

* update comments.
* update error messages.
fix wrong place holder of template string
remove trailing comma