From MozillaZine Knowledge Base

This article covers uninstalling extensions in Mozilla Suite and SeaMonkey (1.x versions). SeaMonkey 2 (not yet released), Firefox, and Thunderbird users should read Uninstalling add-ons.

Uninstaller extensions

• Mnenhy. This extension includes a chrome manager which lets you uninstall individual extensions.
• ExtensionManager (formerly Extension Uninstaller) by Jeremy Gillick, an extension built from the ground up to uninstall extensions.

Uninstalling manually

Before you manually uninstall extensions it's a good idea to make backups of your profile folder before deleting or changing anything, in case something goes wrong.

Removing profile-installed extensions

You can effectively uninstall all Mozilla Suite or SeaMonkey extensions by opening the profile folder and deleting or renaming the chrome folder (Windows users, read this for help finding it). This will also remove any added themes. [1] Caution: There is also a chrome folder in the program installation directory; do not delete it by mistake.

Alternately, you can create a new profile then transfer your data to the new profile and reinstall any needed extensions or themes.

Removing globally-installed extensions

In rare cases, extensions may be installed globally (that is, in their program installation directory and not their profile folder). Reinstall Mozilla Suite/SeaMonkey to remove all globally-installed extensions.

Related bug report

• Bug 272429 – Backport FF/TB extension mechanism [Extension Manager (EM)] to Seamonkey