A window that popped up and said: "such and such object failed." would be really cool. I use k-meleon solely for making my javascript ie4+ / mozilla compatible.
Not the orrible mozilla javascript console - just a dialog that pops up - ideally an interactive one that will print out all the properties of an object, eg
# print window
window.document
window.left
window.top
etc etc etc.
Please - you will make mozilla easier for developers to support and promote linux thin clients. You'll save the world.