Review for DownForEveryoneOrJustMe by ForXmission
Rated 5 out of 5 stars
Where you Select from your search engines dropdown menu at the bottom | Select Manage Search engines | click one you want | click remove button et viola!
If you download this add-on , you will see the installer here is an XML file not an XPI file like other add-on's, in essence it's a Search Engine, so you deal with it in your Search Engines.
Not sure why it's even listed as an add-on but, what am I to know, it works, and you can also uninstall it just like I said.
To create your own collections, you must have a Mozilla Add-ons account.