What caused me to be logged out of Facebook on Firefox when logging out on Opera?

Sometimes you just have to log in again – a security feature

I do understand that but i was logged in just fine and using Facebook on Firefox just fine until i hit the logout button on Opera.

I think that the facebook login/logout method includes sending some kind of machine ID back to FB. When you logged out, even from a different browser, on the same machine, Facebook thinks that you, on that machine, are logged out.

What @dabbler said is not what happens…

What might have happened, is not that logging out in Opera caused you to be logged out in Firefox, but that logging in did. Whether Facebook actually implemented this is mere speculation*, but it’s technically possible and it makes sense security-wise – if you’re using a desktop browser to access Facebook, you’re probably not somewhere else at the same time. Thus, the other session might be someone impersonating you.

Also, a nice tip: when you’re viewing your profile, you can click the ellipsis (…) on your header image, and then View As… to see what your profile looks like as someone else.

* In fact, Facebook’s security measures are far more elaborate, but something like this could very well be part of it.

Also of its on the same PC, out could be that Firefox and opera are accessing the same cookies, which is how websites keep you logged in every time you close and reopen the browser.

No, Firefox and Opera do not access the same cookies.

Actually, @Vincentt I had given that a thought but I was logged in and using FB on both just fine until I decided to log out on Opera, suddenly finding myself logged out on Firefox.