I'm not able to read any Wikimedia website on Firefox (no problem with Chromium).
Error message : Invalid OCSP signing certificate in OCSP response. (Error code: sec_error_ocsp_invalid_signing_cert).
WereSpielChequers has raised this bug too, and reported it on wikimania-l.
Indeed. Must have fluffed itself up during the past 5 or 10 minutes. Interestingly I disabled OCSP-certificate checks in my browser and still had this issue.
I can confirm that I also had this problem with the Firefox security option "Query OCSP responder servers to confirm the current validity of certificates" enabled (by default). After about a half hour it stopped happening. It now appears that all is well.
We should be back to 100% ok on this issue from the user POV. Mostly-ok was about 15:10 UTC, with some intermittent issues until 16:42. Incident report here: https://wikitech.wikimedia.org/wiki/Incident_documentation/20150820-OCSP