The patch I've just attached for this is a very rough, but working(ish) PoC. You can have an add-on create a column and add string values to it. It is hidden by default at the moment - that probably needs some poking to see where values are being stored and where we can see if the add-on is being uninstalled/disabled etc. There's a little bit of delay in adding values, but we can't do much about that - essentially we're using asynchronous functions for filling what's been designed as a synchronous object. Also need to think about the option for the extension to trigger update of a row/column/everything, e.g. if a value has now changed. Example of its use from the WebExtension side: https://github.com/Standard8/thunderbird-conversations/blob/5a736d9afa0ba348ce9dfe47347c8b57011f14f4/addon/uiHandler.js#L19-L33
Bug 1615801 Comment 5 Edit History
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The patch I've just attached for this is a very rough, but working(ish) PoC. You can have an add-on create a column and add string values to it. It is hidden by default at the moment - that probably needs some poking to see where values are being stored and where we can see if the add-on is being uninstalled/disabled etc. There's a little bit of delay in adding values, but we can't do much about that - essentially we're using asynchronous functions for filling what's been designed as a synchronous object. Also need to think about the option for the extension to trigger update of a row/column/everything, e.g. if a value has now changed. Example of its use from the WebExtension side: https://github.com/Standard8/thunderbird-conversations/blob/95cace58e1623e8aa89c817d5d8166dae2dc7ad9/addon/uiHandler.js#L19-L43