Can't login to forums with Windows XP

[stretch4]

Hello. I'm dual-booting a computer with Xubuntu & Windows XP. When I'm using Xubuntu, the site works fine. But, if I boot into Windows XP I can't login to the site.

I click the Log In link at the top of the page, and it redirects me to sign in with SSO, which I do. SSO logs in and asks if I want to use my real name, email address. I click OK, and then when it redirects me back to the forums I'm not logged in.

I'm using Firefox on both OS's. I don't remember which version of Firefox is on XP, but it just updated this morning. I've been having this problem for weeks now.

[Iowan]

As I recall, I had a similar problem with IE (on XP).
I think I ended up setting the forums as a trusted site.