http://getfirebug.com/releases/firebug/2.0/firebug-2.0.18.xpi
so doing the: about:config
search and double click this entry to change it to:
xpinstall.signatures.required = false
DOES NOT HELP in Firefox 52 (even in developer edition)
A few other differences in how the Inspect feature works:In Firebug (on a Mac, anyway) I was able to put a separate "Inspect" button in the Mac's Dock (similar to Windows Taskbar).
There doesn't seem to be any separate Inspect button in DevTools.
Another difference -- not sure I remember this correctly because I haven't used Firebug in a while -- but I seem to recall that the effect of clicking Firebug's Inspect tool on something in my main web page (not in the HTML code) was quite persistent. In other words, I could click something on the web page, and then cruise around in the panel/code views and get different kinds of information about the selected object. In DevTools, the Inspector-selected object seems to deselect itself quickly so I have to frequently re-select it, which is annoying. It's not "persistent" enough.
Finally... correct me if I'm wrong, but I don't recall Inspect in Firebug being a separate mode. I recall it being just a tool that could work across more than one panel mode. However, in DevTools, Inspect seems to be a completely separate panel (with HTML and CSS sub-panels). This makes no sense to me -- why is Inspect a separate panel? You might want to Inspect a page element for HTML, or CSS, or any other panel, and these are panels that already exist (often in more powerful form) separately. Why duplicate them? Why isn't the DevTools Inspect tool universal?
In sum, like other people here, I found the Firebug Inspector much more usable.