A JSONLZ4 file is a compressed bookmark backup file created by Mozilla Firefox, a free cross-platform web browser. It contains data describing all of the bookmarks used by the Firefox user profile that has been compressed with LZ4 compression.

The JSONLZ4 file is created by Firefox to back up bookmark information that can be restored in the web browser to recover bookmarks lost due to a browser version update. You can find JSONLZ4 files in the "bookmarkbackups" folder, which is located in the Firefox user profile directory. The name of the file indicates the day it was created. You can open, edit, and save JSONLZ4 files using Firefox with the "mozlz4-edit" add-on installed. You can also open and convert JSONLZ4 files with the Firefox Bookmark Backup Reader/Decompressor online program.