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Needs to be updated with new url. But you can edit demonoid.xml manually, it'll be in ~/.mozilla/firefox/<some-folder-unique-to-your-install?>/searchplugins in linux systems. There's plenty of characters that look like garbage, but the important part to change is at the end of the file and should look something like the following:
<os:Url>template="http://www.demonoid.pw/files/?query={searchTerms}"</os:Url>

After that change save the file and restart Firefox.