if this happens only for one mail-account in TB , then settings in that mail-account has changed , you need to re-enter/re-verify all login-credentials, mail-server name/address,etc info. if this happens for any mail-account in TB , then very likely an AV/FW/SS software causing this problem . see below link on AV. also check in new TB , if your address-book is working/available or not. More/Related Info: • [https://support.mozilla.org/en-US/questions/1315734#answer-1373915 Disable AV/FW/SS software] mail-protection OR enable+add+allow EXCEPTIONS in it & add mail-server names/address . Some AV/SS creates insecure proxy/gateway for scanning/checking emails, but there are other secure way to scan/check emails . Please make sure the option "Allow AV to Quarantine email" is selected inside TB, and make sure your AV/SS is set to "Scan all files on access". • [https://stackoverflow.com/a/64980413/3553808 Disable auto-update or update of TB] ''(please upvote my Q+A both, inside the linked page, if its useful/helpful)''.