AdwCleaner 7.1.1. I think this is a false alarm:

***** [ Firefox URLs ] *****
PUP.Optional.Legacy             suggests.go.mail.ru

"C:\Program Files (x86)\Mozilla Firefox\browser\omni.ja"

mailru.xml

<!-- This Source Code Form is subject to the terms of the Mozilla Public
   - License, v. 2.0. If a copy of the MPL was not distributed with this
   - file, You can obtain one at http://mozilla.org/MPL/2.0/. -->

<SearchPlugin xmlns="http://www.mozilla.org/2006/browser/search/">

[skip]

<Url type="application/x-suggestions+json" method="GET" template="https://suggests.go.mail.ru/ff3">
  <Param name="q" value="{searchTerms}"/>
  <Param name="gp" value="900200"/>
</Url>

[skip]

</SearchPlugin>