MozRepl enables you to directly evaluate JS code written in Emacs in Mozilla — without any reloading.

From the README:

This extension lets you write Javascript code in Emacs and have it evaluated in a running Mozilla immediately. You will be able to:

  • Send lines of code from an interaction buffer.
  • Send regions of code from Javascript sources.
  • Send the block the cursor is in (a function or other brace-delimited area) from Javascript sources.

via the XUL Weblog.