Give it a try: https://developer.mozilla.org/en-US/docs/Tools/Migrating_fro... and https://developer.mozilla.org/en-US/docs/Tools
I still miss the old opera dev features that offered way better and advanced features years ago.
You mean JS code in the debugger? You may want to raise an issue for this in the new debugger which seems much faster developement progress: https://github.com/devtools-html/debugger.html
Or html in Inspector or CSS in the rule view?
Just checked and firebug is still here and works and will be supported until firefox 52 when firefox moves to become a multiprocess program. The firebug team is working to integrate firebug into firefox.next because it would be too complex to port firebug to a multi process architecture.
I didn't know about this and am not happy about it, I guess this means I switch to firefox esr and pale moon to continue using firebug if the dev tools are not to my taste (as they are now and have been since their introduction)