Body does not fill wrapper in Firefox

I’m working on https://www.BigBlackMeatyRaindrops.com for a friend of mine. I’m using the Benevolence theme by Thought Mechanics. In IE it looks fine, but in Firefox, there is a space between the masthead and the body, AND the body and the footer, in which you can see the background image.

CSS is here: https://bigblackmeatyraindrops.com/blog/wp-content/themes/benevolence/style.css

How can I fix the CSS so that the body and/or footer expands to fill the gaps in Firefox?

Thanks!
Elisabeth Chaney
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