Is there a way to make the MicroB Browser open up a custom homepage?

I don't like the default config where MicroB opens up the bookmarks window, which then annoyingly stays open after choosing a bookmark which opens in another window.

I have looked at some simular threads but they did not really provide a solution....

I have also tried about:config and edited:
browser.startup.homepage = my homepage choice
browser.startup.use_default_homepage = true

But that does nothing as far as I can see.....

Any suggestions?