Copying someone else's description of the bug, which matches mine:

So I am running iceweasel (libre/iceweasel-1:49.0.deb4-2) and the default search engine is set to searx.laquadrature.net. However, I would like to instead use searx.me as it is nicer to use.

When I try to install a search plugin for it via addons.mozilla.org however, I get the following error: 'Iceweasel could not install the search plugin from “https://addons.mozilla.org/firefox/downloads/latest/searx-0-9-0/addon-713044-latest.xml?src=ss” because an engine with the same name already exists.'

Similarly, if I delete the search engine from 'One-click search engines' and visit searx.me, then use the search bar dropdown box to 'Add searx' I get the error: 'Iceweasel could not install the search plugin from “https://searx.me/opensearch.xml” because an engine with the same name already exists.'

I would presume both of these errors could be resolved by deleting the default search engine configuration in the profile directory but I can't find anywhere where it would be listed.