If you are seeing a message when using a Power-Up that you aren't able to authenticate because your browser's localStorage is disabled, this article shows how to fix that in Chrome.
In the screenshot above, the [*.]trello.com domain will allow localStorage for all Power-Ups made by Trello. Make sure to have the checkbox Including third-party cookies on this site ticked and click "Add".
To make these changes in Firefox, you can go to "Settings" > "Privacy & Security" > "Manage Exceptions..." in the Cookies and Site Data section. There, you can add an exception for the service you want to allow, though you won't be able to use wildcards as in Chrome, so you need to be explicit in the correct address, such as "https://github.trello.com."
Additionally, Firefox will default to http, so you also need to specify https for the exception to work.
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