Both .jsonlz4 and .json files are used by FireFox to store bookmarks. The difference is that .jsonlz4 files are auto backups compressed using LZ4 compression, while the bookmarks saved by users are in .json format.
What can happen is that both of these files can be corrupted and sometimes recovery is needed and why users need to turn .jsonlz4 files to .json file, which is what jsonlz4 to json conversion pretty much represents.
Your best option how to handle this is to use online recovery tools like the Firefox Bookmark Backup Reader/Decompressor, with which you can recover compressed .jsonlz4 autobackup bookmark files to the HTML version and import that back to FireFox to restore the bookmarks.