No, you turn off apps from unknown sources, then you download Downloader. You can then use Downloader to download apps from any internet url. You can do this all from the Firestick’s home screen.
Here is a sample tutorial:
https://www.firesticktricks.com/sideload-apps-on-firestick.html
Firestick can run FOSS apps. You first turn off unknown sources blocking, and then use Downloader to sideload FOSS apps like Kodi.
Another option is you can buy a thin client with an HDMI port for like $100, velcro it to the back of your TV, and install a light linux distro on it. Or you could try doing it with a raspberry pi, or similar low powered device.
That’s the tradeoff for FOSS, though. Official Netflix apps and Edge are not FOSS, and Netflix in general does not seem willing to let FOSS browsers/apps stream in 4k.
The thing is that OP seems to want FOSS apps but stream from commercial companies, and there is a bit of tension between the two goals.