You (perhaps understandably) misread the reference on the SpamCop home page http://www.spamcop.net/ -- it doesn't mean that it is approved by Firefox, it means that Firefox is approved for use with the SpamCop web site! <smile>
You also (also understandably, I would guess) misunderstood what SpamCop did. SpamCop makes available a "blacklist" of IP addresses from which spam has been seen to originate. Subscribing e-mail providers sometimes use that list to REJECT incoming e-mail (against SpamCop's own recommendation), so what is likely happening is that the e-mail provider of one or more of the people to whom you are sending e-mail is rejecting your e-mail because the IP address through which your e-mails are being sent is or was on the SCBL (SpamCop blacklist) or are "blaming" the SCBL when they are really using some other criteria.
...What to do about this? First, you should put pressure on your e-mail provider to ensure that they act on SpamCop reports of spam, if they are receiving such reports. If they aren't, they should contact spamcop (email@example.com) or visit the SpamCop FAQ (http://forum.spamcop.net/forums/index.php?showtopic=2238) to learn what they can do. If they are unable or unwilling to correct the problem, you should consider changing e-mail providers. Second, if you have an alternate way of contacting those to whom you are not able to get e-mail (and since e-mail is not a guaranteed-delivery mechanism, that would be a very good idea in any event), you could ask them to whitelist your e-mail address, if their providers allow that. Third, if those providers who are rejecting based on the SCBL are including the offending IP address in their rejection notices, visit the SCBL lookup facility (http://www.spamcop.net/bl.shtml) and enter the IP address to verify that it is listed and, if not (or they are not including the offending IP address in their rejection notice), contact the rejecting provider and ask that they correct their message and/ or their copy of the SCBL -- it changes minute-by-minute as spam sources are added and removed.