Blogger Bar - Search Blogger, â€œBlog Thisâ€ function for any page you're on.
Clipmarks - Clip any portion of a website to save, email or put on your blog.
Deepest Sender - Blog to multiple platforms from one window.
JustBlogIt - Just right click, and you're blogging about the page you're viewing.
LiveJournal Addons - Notifies you of friends updates, messages and more.
LiveJournal Hook - Gives you quick access to many LiveJournal commands and tags.
MySpace Blog Editor - Post directly from Firefox, drag and drop clips from sites.
Scribefire - A blog editor that integrates with your browser and allows you to drag and drop items from a web page into a blog post.
Twitterbar - Post to Twitter directly from the address bar.
WordPress.com Sidebar - Adds a quick access sidebar to get to your WordPress.com account.
Uploading Photos and Media
Fireflix - Sidebar extension to upload, browse, and manage your Flickr photos.
Firefox Companion for Kodak EasyShare Gallery - Upload photos directly to your Kodak EasyShare Gallery.
Firefox Universal Uploader - Universal program for uploading to sites such as Flickr, Picasa, YouTube, and Facebook.
FireFTP - An integrated FTP system that syncs and checks versions.
Flickr Sidebar - View the pictures in your account in a sidebar.
Fotofox - Select, tag, and upload images to popular sites such as Flickr.
ImageBot - Lets you upload photos to ImageShack or Photobucket.
OakFlickr4PFF - Allows uploading to your Flickr account.
Adsense Notifier - Displays your current earnings from Google Adsense in the status bar.
Copy as HTML Link - Copies the address of a page to the clipboard as an HTML ready link.
Create short URLs - Right click an address, click wrapURL, and create a shortened URL for your use.
Dict - Highlight a word, right click it, select define and get a definition in a pop-up window.
DictionarySearch - Similar to Dict, but the result appears in a new tab.
FastDict - Add quick access to any number of dictionary sites to spruce up your blog entries.
ShrinkThisLink Link Shrinker - Use the ShrinkThisLink service without leaving the page.
Tiny.pl - Shortens an address via the tiny.pl service.
Blog Design, Customization and Development
ColorZilla - Get a color reading from any spot in your browser.
DT Whois - Adds a button for Domaintools.com whois service for the site you are currently viewing.
Firebug - Look under the hood of your favorite websites and take a peak at their code.
Greasemonkey - Allows you to run small java scripts to manipulate the way a page displays to you.
HTML Validator - Checks the validity of a page's HTML and tells you the results in simple graphics.
IE Tab - Lets you view a webpage in an Internet Explorer tab without leaving Firefox.
iWEBTOOL Webmaster's Toolbar - Allows you quick access to services and tools from iWEBTOOL.
KGen - Allows you to see the strongest keywords for a given site.
LinkChecker - Checks the validity of all links on a given page.
Load Time Analyzer - Allows developers to measure how long a page takes to load.
MeasureIt - Draws out a ruler to get a pixel dimension of any element on a page.
MetaTags - Checks the meta information of a site and features an automated search engine submission tool.
RankQuest SEO Toolbar - Quick access to over 30 SEO tools.
SearchStatus - Keyword density analyser and checks backlinks.
SEOHAND - Tracks a site's standings and creates graphs over time.
SEOQuake - Ever wonder why a page gets good search ranking? This will help you figure it out.
Web Developer - A highly regarded debugging tool for designers. (via)