Blog posts get chopped in Firefox browser

Hello,

My blog https://www.sweetsapling.com looks good on every browser except Firefox. I don’t know code at all so could someone tell me EXACTLY what I need to do to fix this?

Also, is it possible to adjust the width of the Bueno template? I would like the header image to be a little bit wider so I can make the body/post area to be a little bit wider.

Thank you so much for the help! I really appreciate it.

The blog I need help with is https://www.sweetsapling.com.