Is it possible to open the Firefox interface in the DOM inspector?

Question

Browsers

I wanted to fix some little things in one of the FF extensions, I found ordinary CSS there:

.tab-icon-image,
.tab-throbber {
  max-height: 16px;
}

Blind editing is very inconvenient, you have to restart the browser 20 times, and in general - I want to look at the structure, highlight the element with the cursor (in my case, the browser tab), see its CSS properties and change them on the fly, immediately looking at result. I did not find, unfortunately, such extensions. Tell me, is it possible to somehow edit the browser interface on the fly (for example, using Firebug)?