As XUL developer, MozRepl is definitely my favourite Firefox AddOn, an open door from my emacs to my firefox development profile.
Using MozRepl from emacs is simple and agile thanks to an Emacs minor-mode
which connect an emacs buffer to a MozRepl session, over a tcp telnet-like connection.
I use MozRepl for real-time interaction session during talks and
“emacs-addicted” so they lost the interactive workflow experienced during this sessions.
Some months ago I’ve assembled a little experimental extension which embed a MozRepl
session in a XUL panel.
MozRepl in a Panel adds a Tool menupopup in Firefox, Thunderbird and Songbird,
and can be adapted to overlay other XULRunner application with no work.
It can be very useful to debug your extensions or XULRunner applications and to demostrate
scripting features during technical talks.