Nice replacement for the old Context Search add-on which I started to miss bitterly right after Firefox 57 came out. Something like this should REALLY be standard in Firefox.

That said, getting the add-on to work in "Automatic" mode on GNU/Linux (Ubuntu, in my case) was a bit of a hassle. The instructions on the GitHub site could be more helpful here. They put too much focus on the location of ContextSearch.json (no, DON'T put that file or anything else under /usr/lib/ or /usr/lib64/, these are to be maintained by your package manager alone) and leave more important points unmentioned.

To make a long story short, I had to do the following:

(1) Put ContextSearch.json and ContextSearch.py from the git repository into ~/.mozilla/native-messaging-hosts/.

(2) Make ContextSearch.py executable (chmod u+x) AND convert it from CR/LF (Windows) to LF (Unix) line endings so that python can actually run the script. (I also changed the shebang to #!/usr/bin/env python).

(3) Change the path value in ContextSearch.json to the actual location of ContextSearch.py (why doesn't it just default to ContextSearch.json and ContextSearch.py being in the same directory?).

(4) Enter the path of my search.json.mozlz4 file in the add-on's preferences.

And hey not-quite-presto, everything works! Well, apart from a few details, like search engines using POST rather then GET requests not working but showing up in the context menu. All in all, a promising extension with a few rough edges.