Lightning Requires Restart Featured

Organize your schedule and life's important events in a calendar that's fully integrated with your Thunderbird email. Manage multiple calendars, create your daily to do list, invite friends to events, and subscribe to public calendars.

Rated 3 out of 5 stars (1,922)
5,886,184 users

Session Manager

Session Manager saves and restores the state of all or some windows - either when you want it or automatically at startup and after crashes. It can also automatically save the state of open windows individually.

Rated 4 out of 5 stars (486)
281,547 users

ChatZilla Requires Restart

A clean, easy to use and highly extensible Internet Relay Chat (IRC) client.

Rated 4 out of 5 stars (125)
123,136 users


UnMHT allows you to view MHT (MHTML) web archive format files, and save complete web pages, including text and graphics, into a single MHT file in Firefox/SeaMonkey.

Rated 4 out of 5 stars (229)
92,461 users

gContactSync Requires Restart

Synchronizes contacts & groups between Thunderbird or SeaMonkey and Google Contacts. Imports Facebook friends, contacts you follow on Twitter, and more.

Rated 4 out of 5 stars (227)
84,561 users

Send Later Requires Restart

True "Send Later" functionality to schedule the time for sending an email.

Rated 5 out of 5 stars (79)
64,949 users


FoxClocks shows times around the world - or just your local time - at the bottom of your browser. It deals with daylight saving time so you don't have to.

Rated 4 out of 5 stars (327)
60,731 users

Add-ons Manager Context Menu Requires Restart

Add more items to Add-ons Manager context menu.

Rated 5 out of 5 stars (38)
49,587 users

Copy Plain Text 2 Requires Restart

Copies text without formatting

Rated 5 out of 5 stars (69)
43,652 users

Mail Redirect Requires Restart

Allow to redirect (a.k.a. "remail") mail messages to other recipients.

Rated 4 out of 5 stars (131)
36,229 users

Diagnostics for Adblock Plus

Diagnostics tool for Adblock Plus, provides some insight into internal processes of Adblock Plus.

Not yet rated
35,719 users

QuickNote Requires Restart

A simple note-taking extension.

Rated 5 out of 5 stars (174)
27,261 users

Copy As Plain Text Requires Restart

Removes formatting from copied text.

Rated 4 out of 5 stars (59)
19,764 users

SmartTemplate4 Requires Restart

SmartTemplate4 offers an easy way to automatically create customized replies by auto-inserting various mail headers. Templates can be created for every account as well as for the default (all email accounts). Also supports the Stationery Add-on.

Rated 5 out of 5 stars (51)
19,540 users

AutoCopy 2 Requires Restart

Copies selected text to the clipboard automatically.

Rated 5 out of 5 stars (66)
18,601 users

Filter Uploader for Adblock Plus Requires Restart

DOES NOT BLOCK ADS! Don't install this add-on if you don't know what 'uploading' means!

This add-on Uploads the filters from your Adblock Plus installation to a FTP, HTTP, HTTPS server...

Rated 4 out of 5 stars (40)
15,446 users

Quote Colors Requires Restart

Quote Colors allows you to select text and background colors of different quote levels which are applied when reading or printing mail/news messages. Additionally, you can choose between graphical and traditional plain text quoting display mode.

Rated 4 out of 5 stars (44)
13,812 users

Mouse Gestures Suite Requires Restart

Mouse gestures for SeaMonkey & Firefox: navigate & execute commands using mouse gestures (with single mouse button) and rocker gestures (by pressing 2 mouse buttons). Also: scroll wheel navigation, page scrolling, tab scrolling.

Rated 4 out of 5 stars (39)
13,494 users

MinimizeToTray Plus Requires Restart

Note from Mozilla: This add-on has been discontinued. Try MinimizeToTray Revived instead.

Minimize applications to the system tray.

Rated 4 out of 5 stars (193)
12,585 users

Add-on Update Checker Requires Restart

Adds Tools menu item to check Add-ons for updates. Includes toolbar icon.

Rated 4 out of 5 stars (56)
12,207 users