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Firefox Filter Group
After clicking Add Filter Group you'll get this new window where filter groups can be listed. If you want to label the new filter you're creating, double-click the "New Filter Group" text and it will highlight so you can give it a name. (Extra points for more creative names than the one I chose.)
Click the filter group to highlight it, then hustle your mouse over to its Actions drop-down menu to select Show/hide filters.
Previously: Hide Twitter preview images and videos using AdBlock in Chrome (1 page).
In Firefox: Twitter 'Hide Preview' Code
After you clicked Show/hide filters, the window splits into two panes (above). Click the button in the upper right of the window that says Add Filter.
An entry field will appear in the "Enabled: Filter rule" box. That's where you'll paste this code:
(Yes, it's the same code as used in the Chrome/AdBlock hide Twitter previews technique.)
To finish, click Close. And you're done!
Thanks for the tip: @xabean