Opera has just released a competing set of developer tools called Dragonfly. They will be automatically included in Opera 9.5 beta 2 and newer versions of the browser, and you will be able to enable them by going to “Tools > Advanced > Developer Tools”.
The Norwegian software maker is highlighting several innovative features of Dragonfly that could give it a leg up on Firebug, all depending on how well they work.
Dragonfly will make it possible to debug webpages whether they're on your desktop computer or any remote device, like a cellphone, that runs the Opera browser. This has the potential to make development for mobile devices much easier.
A couple downsides: it will only be available for Opera, certainly not one of the most widespread browsers, and it won't initially support in-browser edits to CSS styles. This latter shortcoming is a big one since CSS editing constitutes perhaps the most useful feature of Firebug.