When I access the site from Firefox, it gives me the mobile version, even though I'm on a Mac, and I have accessed the "correct" version from it. When I access it from Safari, it gives me the "correct" version.

Why is this happening?

 

The theme the browser uses is stored in your browser cookies. Thus you can choose a different theme (and the mobile version is just a different theme) for each client and even for each browser on one client.

Switch to "normal version" and it should change it permanently. It's a button in the lower left corner of the window.