SSL Error: Invalid or self-signed certificate

[stricte]

I'm using Magento version 1.9.2.4 with SSL bought from Alpha SSL - not self signed. I'm unable to upload images on Firefox, it work's ok on Chrome.

[chiefair]

Firefox is probably missing the root or intermediate certificate for Alpha SSL

https://www.alphassl.com/support/install-root-certificate.html

[stricte]

Firefox is missing certificate, can you elaborate on that? I don't install certificate on users browser