I think I found a solution, I checked under Settings -> Advanced (like you suggested) within the firefox app and I deselected the 'Open links in apps' option, now when I click on a link that ends with .html it doesn't try to open an external app (mixplorer)
I will probably need to do more searching into the "open with" button not appearing
The setting I spoke of was in MiX but browsers do indeed have settings such as toggle to use internal or external downloader. It seems that is what you found in FF. Thanks for sharing that it worked. As to the "open with" menu option, since that may be a matter of usage rather than development feel free to take an exploration of it to the FAQ with screenshots of your menu and perhaps a step by step description of how you are going about it. First though, check the " buttons" settings.