Upload files/photos/videos to Flickr, Picasa, Youtube, Box.net(1GB of free space), Facebook, Webshots and OmniDrive in this version. Next versions would support other websites like Google Videos etc. You can add multiple files to the queue at a time.

FireFTP: integrated FTP client for Firefox.

Javascript

FireBug — work with DOM, CSS, JS or XHTML in any web page: available for

Firefox.

FireBug Google Group

Documentation

JSView — maximizes “View Source” by showing any embedded .js and/or .css files

PHP

phplangeditor — GUI for translation of language files of your PHP scripts.

SEO

Live Page Rank — displays Google PageRank for the currently visited page.

SEOQuake — Seoquake is aMozilla Firefox extension aimed primarily at helping web masters who deal with search engine optimization and internet promotion of web sites. Seoquake allows to obtain and investigate many important SEO parameters of the internet project under study on the fly, save them for future work, compare them with the results, obtained for other, competitive, projects. Beginner web masters will find SeoQuake useful for estimating effectiveness and competitive ability of their SEO efforts. In the hands of professional SeoQuake will become a powerful and indispensable tool for analyzing optimization and promotion level of internet web-projects.

Validation

Total Validator — Validates HTML v.2.0 to XHTML v.1.1; accessibility (WCAG, 508); checks for broken links, and more!

developer’s website

Web Developer — has localized versions at developer’s website. Chris Pederick has now created versions for different browsers:

Download for

Firefox

XHTML

FireBug — work with DOM, CSS, JS or XHTML in any web page: available for

Firefox.

FireBug Google Group

Documentation