hi karmenbuttke, are you using kaspersky as your security program? if so, please disable the scanning of secure connections like it's described at https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER#w_kaspersky, reboot your system afterwards and see if the issue is still occurring...

Facebook appears to have enabled Brotli (br) encoding for secure connections and Kaspersky doesn't know about this encoding and strips it from the HTTP response headers.

See also:

• https://forum.kaspersky.com/index.php?s=bbdfcf797a91bc90230472c52c4b103e&showtopic=351027