No, there is no web browser app for Roku devices. But you can most certainly use a Roku TV as a computer monitor, assuming your computer has an HDMI output. There is a 43" 4K Roku TV just to my left right now that is my secondary monitor for my work laptop. It's connected to my docking station, but most laptops nowadays have HDMI outputs, as do a lot of desktop computers. In fact, the majority of my "computer" monitors in my home are actually TVs.
The Firefox channel you noted is really just something designed to accept a screen cast from a Firefox browser on a computer. In fact, it's extremely limited, as this is the description:
Watch videos pushed from Firefox for Android on your Roku
So, only for casting videos, and only from an Android device. But you can cast the screen from a Windows 10 laptop, which might provide what you're looking for.
Dan
Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.
