Remix and share web pages instantly (bookmarklet)

You can make Firefox 3 create an automatic HTML backup (bookmarks.html) when you exit Firefox if you set the pref browser.bookmarks.autoExportHTML to true on the about:config page.
That backup is created by default in the profile folder as bookmarks.html, but you can set the file name and path via the pref browser.bookmarks.file on the about:config page.
Note: an HTML backup doesn't preserve tags and annotations, so you lose those if you need to import the HTML backup.

Navigate up one or more URL levels, by clicking on icon in URL bar, or using icon's context menu, or pressing shortcut key.

Looks up a selected word in OS X's Dictionary.app

Shows progress of page load in location bar.

Displays the progress of the current uploads. Useful for sites that does not shows the upload progress like youtube.

Press CMD+Enter in url bar to open a new tab with url bar contents.

Adds a menu to the menubar and the panorama button showing all tabs grouped by tabgroups (panorama/tabcandy groups).

Make any pages' search functionality available in the Search Bar (or "search box")...

Capture the whole page or any portion, annotate it with rectangles, circles, arrows, lines and text, blur sensitive info, one-click upload to share. And more!

Migrates opened tabs to Google Chrome.

Displays an icon in the tooltip or next to the cursor indicating the target of a link.

NumExt numbers your tabs with no limitation on how many tabs you can open.

URL Fixer fixes common typos in URLs entered in the address bar. It will fix errors like "google.con" or "youtube,com". You have the option of being asked to confirm corrections before they are applied.

Reload page automatically when selected local files are changed.

Markdown document viewer.
Allows to visualize formatted markdown documents in browser.
(Works under Mac OS X!)

When keyin a-Z,will auto focus to URL bar.

This 15KB restartless add-on will add a "File" -> "Restart" menu item and a ctrl/cmd + alt + r hotkey to restart FIrefox or Seamonkey.

Dig through URLs with mouse right button click (context-menu).

This extension prevents websites from interfering with your copy and paste actions without waiting for bug 542938 to be fixed. No configuration required, the website will simply no longer notice when you copy or paste text.