Since Firefox 45, search engines are stored in
search.json.mozlz4, as noted on reddit. This file may be found in the profile directory at
$HOME/.mozilla/. The file can be decompressed, edited, then recompressed.
mozlz4 format has a custom header that prevents it from working with standard
lz4 compression utilities. For methods to decompress and recompress them, see How to decompress jsonlz4 files (Firefox bookmark backups) using the command line?
search.json.mozlz4 after editing the
xml files located at
/usr/lib/firefox/distribution/searchplugins/locale/, as Mitch♦ suggests; or deleting them, as Kindly recommends. Firefox will regenerate the
mozlz4 file when it is next started. If the file is not deleted, the changes made to the
xml files will have no effect.
Other solutions that don't appear to work:
Attempting to reinstall the search plugin after deleting it from preferences, as Komalleana suggests, gave me an error stating that the search engine was already installed, even after restarting Firefox and confirming that it did not appear in the list.
Changes to the
xml files are no longer contained in
xul-ext-ubufox, which DK Bose mentions. Some are now in the
firefox package. Others are in the