Well, problem doesn't reproduce anymore, so, I guess bad luck. Thanks for your work.
(in the meantime, did things another way : directly uncompressing search.json.mozlz4, manually editing and recompressing. takes a bit of time but gets things done)
If you open an issue on https://github.com/evilpie/add-custom-search-engine/issues with the Open SearchXML preview (under advanced options) I might be able to help you.
This still works correctly for me. So this might be an issue with the image or your Firefox configuration.
