Under about:config in Firefox, set the "image.animation-mode" to none, and in Vivaldi, go to the picture icon on the very bottom right of the window on desktop, and click the "Never" radio button.
Firefox mobile users can do the same thing. There isn't a mobile option for Vivaldi yet.
@Elizafox you're actually quite inspiring, Eliza. I pinned your thread on ASD onto my SPC meta page.
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