Use Firefox in embedded mode

I have multiple HTTPS sessions configured that require Firefox as the web browser. I currently have these sessions configured to run in external display mode but I want them to run in embedded mode. While changing them, I noticed that Firefox is not listed as a web browser anymore.

While investigating this myself, I stumbled upon this article: Web browser Firefox - Devolutions Documentation which confused me a little. Although it looks for 95% describing my issue, the remark The Firefox web browser is deprecated in embedded mode in the top of the article could also explain what I'm facing. I could not find any official statement about this thoug.

Am I dealing with an issue here or is it by design that Firefox is not available anymore for embedded sessions and is there an alternative other than sticking by external display mode?