Why can't Firefox open .mht email extensions within its default browser? It has never done so to my knowledge. They always point to Internet Explorer or Microsoft Edge. Often an email may not display images correctly and the ability to open them in a web browser is offered, "If there are problems with how this message is displayed, click here to view it in a web browser." This is certainly a development addition when people view their Gmail from Firefox or use Outlook. Not everybody uses APPS. People who don't want Microsoft Edge watching every move they make have made effective permanent changes in removing the deep embedded PUP by Microsoft. Firefox version: 148.0