If the base requirement is to have a GUI, then there are lots of ways to do that which do not involve the browser at all! I've made some pretty fancy GUI's with Tk, but there are other options. I'm just wondering if we may all be pounding away at the wrong "solution"? If the real question is "hey, I want a GUI", then the discussion goes into a whole different direction and probably should be a new thread.

As another server option, one project that I'm working on (not done yet), is to get a Raspberry Pi 3 up and working with an Apache server for my "testing play box". I should be able to do this for less than $100 + brain sweat. This thing actually has enough horsepower to be a full Unix server the size of my palm.

Update: see XY Problem

You are right, I did not have a clear idea of what I wanted... The fact is that I had a .svg image as output of a little program and I found out that browsers can read svg; moreover I had just learned HTML and I wanted to "try it out" to see how it was like. So this idea to use firefox stuck in my mind and I could not think of anything else.

However, it was just a for-fun-program, so I do not care if it is not perfect. Now I think the best solution would be to run a web server, but, actually, it does not worth. Of course I care of my program, but I am satisfied with having to run it manually and open its output in firefox.

I will try not to fall anymore in the XY Problem.

Sounds like you are happy with what you have implemented with Firefox. I did find 21 free SVG Viewers on the web. I don't know anything about any of these, but just mention as other options for you...