If you just want the search plugins you can locate the 'searchplugins' folder within your profile and just copy that somewhere
(There may be some search plugins in the application directory if it's a really old profile or you've edited something in the past)
Profile backup
Search plugins that you've added are in your profile so:
http://support.mozilla.com/en-US/kb/Backing+up+your+information
applies.
If you just want the search plugins you can locate the 'searchplugins' folder within your profile and just copy that somewhere
(There may be some search plugins in the application directory if it's a really old profile or you've edited something in the past)