Editing JavaScript functions on the fly is the main feature I hear devs complain about Firefox devtools (anecdotal). Chrome allows you to edit the JavaScript in the script tag which is amazingly intuitive. I realize editing variable values is possible while debugging, there are console commands and Scratchpad is neat, but it's not the same. Being able to edit the JS directly in the tab, save and see the changes on the page is a huge time saver.

I was able to find a firebug feature request with applicable bugzilla links: https://code.google.com/p/fbug/issues/detail?id=5083

Edit: changed reload to save