User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/601.5.8 (KHTML, like Gecko) Version/9.1 Safari/601.5.8 Steps to reproduce: Since upgrade to Firefox 45.0.2 from Firefox 38.7.1, indexed DB stops working. An UnknownError is thrown whenever I try to use window.indexedDB.open. The console shows below message: IndexedDB UnknownErr: ActorsParent.cpp:592 <unknown> After I delete the local indexeddb storage for http://html5test.com (located in 2ziq4yrx.default/storage/default/http+++html5test.com folder), Everything back to normal. So I believe this is a Firefox bug. The upgraded Firefox failed to read the local storage of http://html5test.com, and then breaks other consumer of indexed db. (There is a similar bug in mozilla.org: https://bugzilla.mozilla.org/show_bug.cgi?id=1264872, but not the same issue). I attached the http+++html5test.com folder. This issue could easily reproduced by unzipping it to <profile>/storage/default/ folder and restart Firefox.

By the way, the folder is not deleted with 'Clear Your Recent History' -> 'Everything', so user can't fix this issue by clear history.

This seems like another case when the user switches back and forth between different Firefox versions. We had to rename a directory in the origin directory: http+++html5test.com/idb/2390841740httsmelt5 became http+++html5test.com/idb/2390841740httsmelt5.files So if the database is created using FF 45, this directory and database file appear in the origin directory: idb/2390841740httsmelt5.files idb/2390841740httsmelt5.sqlite If the database is deleted in JS, both go away. Now if it's created in FF 45 and deleted in FF 38, only .sqlite is deleted, since FF 38 doesn't know anything about the .files directory. When you switch back to FF 45, we fail to open the database, because we've had always a policty to treat unknown files as something unexpected. Similar issues were filed in bugzilla, sharing the same profile by multiple FF versions is not officially supported, but we plan to prevent this from happening somehow in bug 1246615.