A JSONLZ4 file contains compressed user profile data created by Mozilla Firefox, a free cross-platform web browser. It stores various profile data and settings, such as bookmarks, home page configuration, toolbar layout, and saved passwords. JSONLZ4 files are compressed with LZ4 compression.
You will most likely only encounter JSONLZ4 files if you use Firefox to browse the Internet and are exploring the "Profile" folder where Firefox stores information about your user profile. Your user information is saved in a separate place from the Firefox application to protect your user profile from being lost in case you need to uninstall and reinstall Firefox on your computer.
You can navigate to the location of your profile in information by selecting
in Firefox. Then, click in the "Application Basics" section to open a window that displays your profile folder.
The JSONLZ4 files are not meant to be manually opened. Instead, they are referenced by Firefox to load user profile settings. You can delete the files if they become too numerous and large, but you will lose information regarding your user profile.
JSONLZ4 files storing bookmarks are 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.