I don't know if this will help. If not, please let us know.

delete the search.json file and possible search-metadata.json and search.sqlite files in the Firefox profile folder to reset the search engines to the default.

Type about:support in the address bar and press enter.

Under the page logo on the left side you will see Application Basics. Under this find Profile Folder. To it’s right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Locate the files. Then rename or delete them. Restart Firefox.

hhgoten said

Running firefox 38.0 on Xubuntu 15.04 i can't use my own xml file in /usr/lib.../searchplugins/locale/(de,en-US)/ anymore. Managing searchengines/plugins doesn't show my files anymore. Under Firefox 37.0 and Kubuntu 14.10 every selfmade xml file like goo_year.xml was selectable. I can only add new searchengines/plugins from web . For example "Google Default" installs in ...home/.mozilla/firefox/(profile-name)../searchplugins/google-defaults.xml and there I could modify params. I can add &tbs=qdr:y and it works. But why is a copied and renamed abc.xml file not showing up ? Is there now a kind of database instead of only this files ?

Thank you very much Fred. Before that , I tried to rename search.json , but obviously this was not good enough. A file search.sqlite did not exist on my harddisk , but after deleting the other two, all seems to be fine. Even all my selfmade icons are visible and at least for the moment all runs well. By the way - should I avoid automatic update of the search engines/plugins for sure ? Best regards Waldi, Berlin, Germany

I can not say. You will have to keep an eye out for its return.