I log in, then go to a mod page to comment on a mod but it says 'You need to login or register to post.' So I click the login link and it goes to the Curse free membership page, I hit JOIN and it takes me to my profile. So the site says I'm logged in but I can't comment on any mods.

Do you have javascript disabled?  There was a change to the way cookies are used on the site - have you tried logging out and back in?

I have javascript enabled, but i can't even log in using firefox (the login form does not appear), but i can using the IE tab addon for Firefox.

A patch that should stop this problem will be coming out on Monday.

In the meanwhile, please delete all cookies from curse.com, and the cookie from auth.curse.com and re-log in.