I have tried the page in Chromium, and it looks the same as in Firefox (wrong, but the same). Could you attach a few screenshots?

If I make the window smaller to switch to mobile view then the left part of the menu items are hidden because of a margin: -50px; rule (in this case margin-left)

This same margin:-50px; rule seems to be hiding the menu (in this case margin-top)

@Mesko-

You can see the Menu bar beginning with home on the Chrome version, but not the firefox desktop version. On the firefox version, it's hidden in the image at the bottom.

I think it is mainly the margin setting in this rule in style.css (1355) that is causing this:

• https://www.belosoftware.com/home/wp-content/themes/twentyseventeen/style.css?ver=20190507

.main-navigation ul {
	background: #fff;
	list-style: none;
	margin: -50px;
	padding: 0 1.5em;
	text-align: left;
}

This looks better for me:

.main-navigation ul {
	background: #fff;
	list-style: none;
	margin: 0 auto;
	max-width: 700px;
	padding: 0 1.5em;
	text-align: left;
}