Re: Opening https://www.tumblr.com/ gives me the following error : Error code: ssl_error_rx_record_too_long on Mac.
Try to rename the cert8.db file in the Firefox profile folder to cert8.db.old or delete the cert8.db file to remove intermediate certificates that Firefox has stored.
If that helped to solve the problem then you can remove the renamed cert8.db.old file.
Otherwise you can rename (or copy) the cert8.db.old file to cert8.db to restore the previous intermediate certificates.
Firefox will automatically store intermediate certificates when you visit websites that send such a certificate.
If that didn't help then remove or rename secmod.db (secmod.db.old) as well.
You can use this button to go to the Firefox profile folder:
Help > Troubleshooting Information > Profile Directory: Show Folder