User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:220.127.116.11) Gecko/2009070611 Firefox/3.0.12 (.NET CLR 3.5.30729) Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:18.104.22.168) Gecko/2009070611 Firefox/3.0.12 (.NET CLR 3.5.30729) It appears that XmlHttpRequest no longer works with self-signed SSL certificates, and if you choose to ignore the popup, it will automatically fail the request. This worked up until Firefox 3. I know that several people including myself have created in-house extensions that rely on self-signed certificates, because these will never be seen from the outside. We are currently having to stick with Firefox 2.x. Reproducible: Always Steps to Reproduce: Create extension that uses XmlHttpRequest to a server with self-signed certificate. Actual Results: Request fails (invalid certificate) Expected Results: I was expecting there to be an option to ignore SSL errors.