Ask HN: Keep cookies from multiple sites seperate

I usually don't stay logged in accounts important to me very long because I'm paranoid about cross-site tracking using cookies. The solution, I think, would to be to change the way firefox handles cookies, such that I can specify that a firefox tab's cookies be isolated from the rest of my browsing. So, two questions for the HN community:

1. Is keeping old cookies a security risk?

2. How can I keep different sites cookies separate?