Recently on Reddit, Firefox users reported they lost all their browser preferences and add-ons data due to power failure or a sudden Windows crash (BSOD), Mozilla working on the bug since last 4 months, has found that the changes they made in Firefox 67 caused the issue and fixed it. The organization is tracking this bug for Firefox 68 and Firefox 69.
While Firefox is running if you turn off your computer without shutting down by pulling the power plug, the profile and preference file (prefs.js) get corrupted and emptied, results in Firefox to appear as just now installed. The issue is reproducible in current Firefox 67 stable version.
The affected users confirmed to Mozilla in the bug thread that their bookmarks, add-ons and history are intact, but they lost all their customization including the preferences and data of Add-ons.
Note: This is neither related to the Firefox profile issue that asked users to create a new profile when they updated from the previous version nor AVG Antivirus password protection feature that deleted all passwords and logins.
By default, to minimize any data loss, any changes you make to Firefox preferences will be written to Prefs.js file within 500 ms. There still is a possibility for data loss to happen due to power outage or blue screen of death while prefs.js being saved.
Add-on developer Kestrel actively involved in the discussion revealed he found “the time delay before the last CloseFile” is 30-40 ms and is way shorter than 500 ms and confirmed the bug 1478124 has changed file access behavior somehow.
Mozilla prepared the patch and set the issue status as “Resolved Fixed” in Firefox 69, expect this fix to get uplifted Firefox 68 beta and release versions.