To see what it's like in earlier versions of Firefox or in Firefox Beta and Release, see Debugger (before Firefox 52).
If you are getting this version of the Debugger and need to switch back to the old version, you can do so by visiting about:config and setting the "devtools.debugger.new-debugger-frontend" preference to
Searching for files
To search for a particular file, press Control + P (or Command + P on a Mac) and type the search term. The source pane will display a list of all matching files as you type. You can use the up and down arrows to move through the list, and Return to open the file you want:
Searching within a file
To search for a particular substring in the file currently loaded into the source pane, press Control + F (or Command + F on a Mac) while the source pane is focused. Press Return to search. The debugger will highlight all matches: