Uninstalling extensions - SeaMonkey
From MozillaZine Knowledge Base
Extensions in SeaMonkey 2 can be uninstalled in the same way as Firefox and Thunderbird, using the Add-on Manager ("Tools -> Add-on Manager -> Extensions").  See Uninstalling add-ons for more information on removing or disabling add-ons in SeaMonkey 2, Firefox, and Thunderbird.
Applies to Mozilla Suite and SeaMonkey 1.x
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.  Caution: There is also a chrome folder in the program installation directory; do not delete it by mistake.
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 reports