Background
I would like to add crates.io to Firefox Quantum but got an error saying that it cannot add it via https://crates.io/opensearch.xml
So here comes a short note of manually adding a search engine to Firefox Quantum.
Step 1: Extract search.json.mozlz4
Definitions of installed search engines are stored in the file search.json.mozlz4 which is located in your profile folder. Under Linux it’s usually ~/.mozilla/firefox/XXXXXXXX.default. If you’re unsure about it, run firefox -profilemanager (or firefox-developer-edition -profilemanager) to find it out.
To decompress this file, a small Python script MozLz4a compression/decompression utility can do us a favour. Note that as mentioned in the comments, you may need to modify the script a little since Python lz4 has updated its API.
$ python mozlz4a.py -d search.json.mozlz4 search.json $ cat search.json | json_reformat | tee search.json