I had the same issue and I solved it by deleting cache and site preferences . I know the people solved by deleting cert8.db inside the firefox profile folder.

bye

Thank you, cippazza, that did the trick. It's curious, as the problem only arose after I cleared all of my recent history about a week ago, so I didn't think that that would resolve the issue. But it did. Very grateful to you for the input.

Thanks, cippazza. I had this problem after upgrading from Firefox 4 to 8. Changing the name of cert8.db in the user profile folder forced Firefox to rebuild cert8.db, which is much smaller and works!

Hi whisky_jack Thanks for your input. I find that simply deleting the entire History does the trick – easy! What do you mean when you say, "Changing the name of cert8.db in the user profile folder forced Firefox to rebuild cert8.db, which is much smaller and works!"? And how would you do that?

Hey MargaretInOban - Locate the cert8.db file in /Library/Application Support/Firefox/Profiles. With Firefox closed, rename (or just delete) the cert8.db file. The file will be recreated automatically next time you open Firefox. This solution worked for me and I noticed that the new file was only 25KB, smaller than the one I deleted that was 672KB.