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2. Delete files in index.db
OS X (10.9):
(From Finder: Go > Go To Folder, or Shift-Apple-K. Enter ~/Library to show hidden Library folder for your user account)
/Users/{USER}/Library/Application Support/Firefox/Profiles/{PROFILE}/indexedDB
...
or
/Users/{USER}/Library/Application Support/Firefox/Profiles/{CURRENT PROFILE}/storage/persistent/{SITE TO CLEAR}
Windows:
C:\Users\{USER}\AppData\Roaming\Mozilla\Firefox\Profiles\{PROFILE} on Windows.
For additional details see: http://stackoverflow.com/questions/9846013/how-to-view-indexeddb-content-in-firefox
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