Firefox

view Javascript errors — Tools > Error Console

more thorough debugging — Firebug

Internet Explorer

view Javascript errors — double-click on the exclamation-alert icon in the far lower-left corner

more thorough debugging

• IEDevBar (it's only useful for JS debugging starting in IE8)
  • built-in to IE8; press F12 or go to Tools > Developer Tools
• Microsoft Script Editor (more info)
  • separate install
• Microsoft Script Debugger
  • separate install

Safari

view Javascript errors — enable the develop menu, Page > Develop > Show Error Console

more thorough debugging — Web Inspector

Chrome

view Javascript errors — Page > Developer > JavaScript Console

more thorough debugging — Page icon > Developer > Debug JavaScript (it's text-only)

Opera

view Javascript errors — Tools > Advanced > Error Console

more thorough debugging — Dragonfly