Hyperpixel 4.0 smaller resolution

Dear all,

I am using hyperpixel 4 as a screen to my openhabian dashboard. To do that I am running firefox in kiosk mode. This works nicely. However UI is not built for such small screens. so the idea was to reduce the framebuffer_width / framebuffer_height in the /boot/config.txt but then only a part of the screen is used.
How can I rescale this small area to the complete screen again. e.g. running the display with only 240 / 400px

I am sure it i sonly a small setting but I am obvously missing something

I’m not sure if this will help but in PiOS, if you go to
Preferences > Appearance settings > Defaults Tab ,
There are options for Large, Medium, and Small screens.
It changes the text size, window size etc.