We have recently added a bunch of new builtin tools in Firefox (please try Firefox Aurora or Firefox Nightly to see a recent version of these tools). We also redesigned the way we show these tools (screenshot here: http://paulrouget.com/e/toolboxTesting/).
We are now working on defining what should be our next moves, and I'm trying to gather as much feedback as possible.
The current plan is to build a bunch of performance-related tools (see https://groups.google.com/forum/?fromgroups=#!topic/mozilla.dev.developer-tools/L9vfZ1-smUI), improve the user interface (better theme), make the tools remotable (for mobile development), and drastically improve the WebConsole (see an experiment here: http://paulrouget.com/e/jsterm.v2/).
But we really want to get as much feedback as possible to make sure we're going into the right direction.
What do you think we should add/remove/fix in our tools? Anything you miss from Chrome or Opera? Or anything you haven't seen yet you'll like to see part of the browser?
PS: because a lot of people have raised concern about "cluttering" Firefox: we are considering providing some of these tools only as addons, to keep the Firefox DevTools as simple as possible.