The KOCT plugin doesn't work if you are using Firefox in private mode. To replicate: install the KOCT plugin in your firefox browser, set "Run in private windows" to "Allow", then open a new window in private mode, and click on the plugin. Under "Transactions", the plugin displays the following error message: "Unable to open database: InvalidStateError: A mutation operation was attempted on a database that did not allow mutations. Error code: undefined." An easy fix is just to not allow the plugin to be used in private mode (that is, set "Run in private windows" to "Don't allow" while you are configuring the add-on). We should at least add info about that to the manual.
We have run into something similar, but it might be the same cause. I assumed it was a browser limitation: Settings > Privacy & Security > History Never remember history I believe using a private tab/window might have a similar effect, not allowing to keep any stored data?