TL;DR: We're adding a toolbar to Firefox, for developers, which includes a command line. It's a great place to add small tools and experimental features, and we're making it easy to add your own commands.
The toolbar should land in Firefox 16 or 17 and will look like this:
The buttons are useful, but I'm most excited about the commands. We can add an almost unlimited set of commands here without cluttering up the UI, or making things slow, and we've done lots of work to make the command line easy to use.
The challenge: It might be stretching things a little to call the command line a 'platform', and commands 'apps', but the command line still needs commands to be successful. There's a list of commands that we're working on for manipulating Firefox developer tools, and we'll be expanding this list to include system level commands too.
We're planning on a hackathon in a couple of weeks to add to the list of commands and to check that it really as as easy as we think it is to extend. Details soon.
If you want to try it out, it's in Nightly now, you'll need to flip the
devtools.toolbar.enable preference in