In Opera you can simply type in opera:webdatabases
in the address field and delete all the web SQL databases stored on your computer.
How do you do the sam
I figured out how to delete the databases. Windows stores user data separately on a per application basis (on Windows 7 in C:\Users\\AppData). So I found the Firefox Profiles folder in this directory, went to the indexedDB folder and deleted the sqlite files. Then I restarted Firefox and it worked! The full path of Windows 7 is like: C:\Users\\AppData\Roaming\Mozilla\Firefox\Profiles\<*>.default\indexedDB