Dotted border around linked images and texts in Firefox Gmail.

When I click on a linked image or text in my email there's a dotted border/outline appearing around it. I've viewed the same email in gmail using all modern web browsers and it's happening only in Firefox. I'm assuming this can be fixed by zeroing out the outline or border property for <a> focus pseudo selector, but as far as I know Gmail is removing the <head> altogether.

Here are some screenshots of the issue in action:

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