Hi @looktothesouth,
Seems to be working on my Mac’s Firefox as well. Plus, 45 is a bit old, I recommend updating them as most of the update contains vulnerability fixes. I also installed 45 then automatically updated to the latest version, hence, it should update your copies as well. Though, that CSS should work as I tested it too.
As for IE, would you mind updating your theme first? There are IE fixes for flex layout after your current version. Maybe it’s related to that one. Though, I recommend doing the update in a staging (clone of your live site).
You may check this https://wpshout.com/quick-guides/all-in-one-wp-migration/, you can use that plugin for cloning your site. It’s the same as migration, except that you’re keeping the copy of your live site instead of moving it.
Thanks!