Why does firefox show me an invalid OCSP signing certificate in OCSP response?

If you are shown an invalid OCSP signing certificate in OCSP response, it may be due to the OCSP-cert having expired.

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number: 132 (0x84)
    Signature Algorithm: sha256WithRSAEncryption
        Issuer: C=NL, O=Digidentity B.V., CN=Digidentity Services CA - G2
        Validity
            Not Before: Sep 20 10:40:55 2012 GMT
            Not After : Sep 10 10:40:55 2014 GMT
        Subject: C=NL, O=Digidentity B.V., CN=Digidentity OCSP

Code

I used these three commands:
$ openssl s_client -connect www.acm.nl:443 2>&1 < /dev/null | sed -n '/-----BEGIN/,/-----END/p' > acm.pem
$ openssl s_client -connect www.acm.nl:443 -showcerts /dev/null > chain.pembundle
$ openssl ocsp -issuer chain.pembundle -cert acm.pem -url $(openssl x509 -noout -ocsp_uri -in acm.pem)
Response Verify Failure
2676036:error:27069065:OCSP routines:OCSP_basic_verify:certificate verify error:ocsp_vfy.c:126:Verify error:certificate has expired
acm.pem: ERROR: No Status found.
More Detail
Use this instead of third command for more detail:
$ openssl ocsp -issuer chain.pembundle -cert acm.pem -text -url $(openssl x509 -noout -ocsp_uri -in acm.pem)
Link
I copied the commands from here: https://raymii.org/s/articles/OpenSSL_Manually_Verify_a_certificate_against_an_OCSP.html