“Cog” icon -> Mail Settings -> Themes -> Classic. Not sure it’ll help you if there’s broken functionality, but it certainly fixes the broken “look” of the new version.

Failing that, consider Basic HTML Version, which should work on anything unless they’re actively blocking it. To get that, you need to click a link while it loads, but you can do that from home and then click the “Set as default” link to keep it that way. You lose auto-saving of drafts, auto-refreshing when new mail comes in, chat, etc., but it all “works” on just about anything down to IE4. Which is impressive.