What I'd love is to have Firefox dev tools functionality in Chrome.

Firefox highlights the edge lines of the selected element automatically, and in all directions (if I recall correctly). I have to use an extension – VisBug – to achieve that in Chrome.

There's also the fonts window, which lets you in on what fonts are used on the page, and lets you modify the selected element's font settings – size, weight, slant/italic etc.. I pay a lot of attention to typography, so that part is important to me.