SeaMonkey Extensions compatible with version 2.2 for any regular user. If you are missing any, then it is not compatible. Of course, this is not exhaustive list, so visit frequently for any inclusion.
Enables desktop notifications like in Chrome (e.g. Gmail, imo.im) and, if the site doesn't support them, it can notify new messages or updates detecting changes of the title (e.g. Hotmail, Yahoo, Facebook, Twitter). Even if the browser is minimized!
Select a number of links with a rectangle and open them all in new tabs in the same or a new window(s), download them or copy them to the clipboard. You can also lasso checkboxes to quickly check or uncheck them.
Allows for customization of the folder pane. Accounts can be rearranged and the startup folder can be chosen.
Adds Tools menu item to check Add-ons for updates. Includes toolbar icon.
This extension prevents websites from interfering with your copy and paste actions.
Note: The same functionality is now built into Firefox, you can just change the dom.event.clipboardevents.enabled preference manually.
Add a compact menu button to the navigation toolbar, and hide the menu bar.
add icons to buttons/menus/anywhere (CuteMenus - Crystal SVG not required)
A no-clutter image viewer for Firefox that allows you to zoom, rotate and display images/videos against a custom background. ImageTweak is a must-have addon for any photographer or photography-loving user.