Prevent duplicate tabs. Allow only one tab for a url. If you click a link that is already opened in a tab, then existing tab would be focused instead of opening a new tab. This works even across firefox windows.
Duplicate This Tab Requires Restart
This add-on provides the much needed feature update of Tab Duplication to Mozilla Browsers. Features Supported include Duplication of Browser Tabs, Detaching Tabs and Merging All Windows.
Dynamic creation of bookmark-style toolbar buttons which you can customize.
Edit Bookmark Plus Requires Restart
Resize 'Add Bookmark' Popup / 'Edit This Bookmark' Popup, fixes tree selection bug, option to disable popup auto close & Update Url and/or Name of a bookmark with the Url/Title of current page.
Replacement of OpenBook addon
Edit Custom Size Image Requires Restart
Fixes a bug in SeaMonkey Composer and Thunderbird Message Compose whereby the Image Properties dialog would always reset the image to actual size.
Email this page Requires Restart
Simple SeaMonkey Email this page button
Enhanced Priority Display Requires Restart
Enhanced thread pane display of message priorities.
Export All Certificates Requires Restart
Adds an option to export all root certificates.
F.B. Purity hides the ads, app spam and other annoying clutter from your facebook news feed, lets you increase the font size, change colours plus many more features and generally makes Facebook a lot less annoying to use.
FAYT Revived Requires Restart
Regain possesion of your good "Find As You Type" search bar in Firefox and SeaMonkey!
Fierr Requires Restart
Replaces the error page with a more stylish appearance with additional features.
Firesizer Requires Restart
Allows you to resize the window to specific dimensions.
NOTE: Users of Firefox 29 and above must install "The Addon Bar" extension to use this, as Firesizer uses the add-on bar that has been removed in Firefox 29.
FireTray Requires Restart
A system tray extension for Linux and Windows, allowing to set up custom icon, hiding to tray instead of closing, display the number of unread messages in mailing applications, and other features ...
FlameFox 3 Rebrander Requires Restart
Rebrand SeaMonkey as Firefox 3 (by 2008WindowsVista and a11RyanC2)
Flexible Sidebar allows to change sidebar width by dragging the handles.
FLST (Firefox 4+) Requires Restart
Provides Focus Last Selected Tab and Tab flipping functionality (based on the FLST extensions by Gorgias and Gozer, but ported to Firefox 4/.../56 and repaired Tab flipping functionality). Enjoy!
Folder Pane Cell Selection Type Requires Restart
Since 20070806 the MailNews folder pane has switched from its previous cell text selection to the full row selection which the Mozilla Suite used to use.
FolderFlags Requires Restart
Allows users to edit the flags Thunderbird sets on folders.
Forecastfox Weather Requires Restart
Get international weather forecasts from AccuWeather.com and display them in any toolbar or statusbar with this highly customizable and unobtrusive extension.
Save better history for fragment URLs.