Cookies disregarding when using another OS.

[Toppy]

Hi,

This question is unrelated to Proxo, but to Firefox.

I use Firefox as my main browser in both of my OS's (XP and Win7), using the FF add-on FEBE, I make copies of my bookmarks, cookies, usernames&passwords, etc... from my main FF in XP, when I use Win7, I restore all of these with FEBE.
But the problem : I keep being logged in under XP, but somehow in Win7 I always to re-login the first time accessing forums, etc....
Mostly the login doesn't even work, and I first have to delete all the site related cookies before being able to login properly.

Is this problem normal, perhaps sites track the OS your using and if they detect you're using another OS, the cookies get worthless and they force you to re-login ?

Has anyone else experienced these symptoms ?

Thanks.
Toppy.