The best replacement for firebug is web developer.

The best alternatives to firebug are rated as following:
★★★★★  web developer
★★★★  fiddler
★★★★  firebug lite
★★★  venkman
★★★  firefox dev tools
★★★  inspect
★★  firefox web developer
★  firefox tools
★  web developer toolbar
★  firefox developer
★  web developer tools
★  mozilla developer tools
★  webkit inspector
  google developer tools
  web inspector
  developer tools
  ie developer tools

Your alternative for firebug is not listed? Suggest!
Speaking about firebug, one probably shouldn't forget about firefox.

Other search terms related to firebug and their ratings are:
★★★★★ firefox
★★★ yslow
★ firefox 3
mafia wars
You have in mind a companion for firebug and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of firebug in all years. Click on graph for more.
firebug
web developer
fiddler