How can I unblock cookies that have been blocked on Firefox? Thank you.

I wanted to block pop-up windows on Firefox, and removed many cookies. It turns out that some of those cookies shouldn't have been blocked, because now I can't open some of my e-mails. I think there must be cookies that I can't see what they are to allow them. How can I retrieve cookies that I have blocked but want back? Or have Firefox ask me from now on if I want them or not?
I have gone to the Edit---> Preferences---> Content then Block Pop-up Windows - Exceptions to try to allow some, but I can't allow them if I don't know what they are.
Thank you for your help on this.
Jeanne Olson
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