Due to recent policy changes on Firefox relating to the new tab, we have had to temporarily remove our extension until we find a solution. In the meantime, please use the following workaround:
1. Download the New Tab Override extension
2. Open a new tab or go over to the extension's settings in the top-right corner
3. Under "URL", paste the following link: https://app.opentabs.org/tabbing
And that’s it, you’re all set to start opening tabs for a good cause!