The latest release of Firebug improves the add-on’s stability, adds several new features and makes Firebug compatible with the soon-to-be-released Firefox 3.6, which is expected next week.
Among the updates in Firebug 1.5 are a much-improved Inspector tool, which allows you to select any element on the page and see the HTML and any applied CSS rules; and several major new features for the Net panel, which can debug XMLHttpRequests. For a complete list of everything new in Firebug 1.5 be sure to check out the release notes on the Firebug wiki.
To grab a copy of Firebug 1.5, you can head to the Get Firebug site, or wait for the update to show up on the Mozilla add-ons website. The Firebug blog says the latest release should be on the Mozilla site next week (if you want to try Firebug betas or stay at the bleeding edge of Firebug development, we recommend installing the add-on through the Firebug site).
Firebug works wherever Firefox does, but be aware that, if you’re using Firefox on 64-bit versions of Linux, some users have reported problems with Firebug 1.5.