> Firefox doesn't resize small enough to fit on the screen.

What exactly does this mean?

* I can make the FF desktop window super tiny

* I can do browser zooming down to 30%

* even if the chrome would obscure the webpage viewport I can go into fullscreen mode

* FF itself has a responsive design mode and ships with Iphone and other small form factors in the dropdown

I understand that it may be easier to tweak Gnome's browser for adding navigation gestures/url bar appearance, etc. But I can't imagine how FF wouldn't be able to display a web page in a way that fits the Librem 5 screen.

It can display a web page, but there's a long way from that to making the UI pleasurable to use on small touchscreen.

Can I ask what you mean by that? Firefox's core has been used on the mobile phone in multiple applications that function just fine and Firefox's desktop version is perfectly capable of resizing down to mobile phone sizes, even emulating a phone if you want.

There's no mobile version of Firefox that would work on GNU/Linux. There was one, but got ditched when Fennec got rewritten into being a native Android app.

Does that mean no extensions and thus no ad blocker? If so, do they have another ad block solution at the device level?

Epiphany has support for Easylist-based adblocking -- at least on the desktop version, I don't know about the mobile interface.

There's no distinction between the "desktop" and "mobile" versions of GNOME Web. It's literally the same code (compiled for the appropriate processor architecture), just at a different window size.

Since I am personally unfamiliar with GNOME’s toolchain for compiling apps for both mobile and desktop, I didn’t want to speak too confidently about the matter above.

So, all options shown in the preferences of the desktop version will automatically be shown in the preferences of the mobile version?

You can test it by shrinking the Epiphany's window while running its latest version on desktop.

Epiphany has a built-in adblocker (using normal adblock style rules).