I started a project[1] a while back when it became clear that the underlying architecture for the new devtools in Firefox was going to be as problematic as Firebug's had been. Longtime Mozilla contributor Wladimir Palant just published a post a few days ago touching on some of the problems with devtools that he ran into when trying to work with it.[2]
The idea with sepsis-inspector is that it should be able to replace Hewitt's old DOM Inspector codebase and be stupid simple to extend with third-party viewers. (For anyone unfamiliar, DOMi has the ability to inspect not just webpages, but e.g. the Firefox UI [and Thunderbird, and SeaMonkey] and extensions as well.)
The interface between the Inspector itself and the viewers is nice and modular for the most part. It's basically Unix pipes in GUI form, with a few services for sugaring. Altogether, this makes its design a good foundation for a generalized inspector framework. (And I do mean generalized. Let's debug PNGs, PDFs, and Haxe, too, but I'm not even talking just the Web here. Want to use it to debug Gnome Shell or your Mono app? Sure. I mean, why shouldn't we be able to?)
Since DOM Inspector, Firebug, and the new Firefox devtools exist, I've focused most of my time on getting the architecture right and then teaching the Inspector how to do some things that those tools can't already do, rather than necessarily shooting for parity from the start. This means that if you download the sepsis-inspector add-on[3] right now, you'll find that it's missing some things that existing tools can do, but at this point, it's just a matter of layering those features on. There's still work to be done underneath, though.
The sepsis-inspector's dependency on XUL needs to go away, for example.
Allen Wirfs-Brock did some research on JS mirrors[4] a few years ago, although nothing seems to have come of it, AFAICS. Everyone decided to roll their own debugging/reflection APIs, I guess. But it's blindingly obvious to me that something based on mirrors is what needs to be powering our inspectors, and that your chosen inspector should be able to do a lot more tricks than what the current crop of web inspectors can do (including, as I said before, doing more than just debugging CSS, JS, and the DOM).
If anyone is interested in collaborating here, you'll find that, for the most part, the sepsis-inspector code is extremely well commented for a Mozilla add-on. Patches wanted.[5][6]
1. https://wiki.mozilla.org/Sepsis
2. https://palant.de/2015/07/17/javascript-deobfuscator-reloade...
3. https://addons.mozilla.org/en-US/firefox/addon/sepsis-inspec...
4. http://www.wirfs-brock.com/allen/posts/228
5. https://gitlab.com/crussell/sepsis-inspector
6. http://www.colbyrussell.com/2013/08/06/patches-wanted.html