Firebug 1.5 is here with new and improved features developers and designers will be very happy about:

Enhanced Inspector
More accurate Net panel timings
Break on HTML mutation
MathML and SVG namespace support
Break on XHR
Improved HTML editing
Persist buttons on Console and Net panel
Separate Computed CSS and Style subpanels
Many many bugfixes and stability improvements

Read the release notes [...]

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Oi cool! Firefox 3.5 is out!
Promising improved performance and support for new technologies, the site lists some new features, that are beneficial to users and web developers alike:
* Available in more than 70 languages
* Support for the HTML5 and elements
* Improved tools for controlling your private data, including a Private Browsing [...]

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