Firefox is a bit picky about intermediate certificates. For best results, add the bundle file to your server so it will send the intermediate certificate instead of assuming the browser will already have it. This article list may help with the directions for your particular server: https://support.comodo.com/index.php?/Default/Knowledgebase/List/Index/37

Firefox automatically stores intermediate certificates that servers send in the Certificate Manager for future use. Stored intermediate certificates show as "Software Security Device" in the "Security Device" column in the Certificate Manager. A server needs to send the full certificate chain that includes all required intermediate certificates. If a server doesn't send a full certificate chain then you wouldn't get an untrusted error when Firefox has stored missing intermediate certificates from visiting a server in the past that has send it, but you do get an untrusted error if this intermediate certificate isn't stored yet.

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