The theme adjusts its layout to fit the screen when the site is viewed on screens narrower than the configured site width.
If the menu content is longer than the available space but the screen is still large enough that the mobile menu is not triggered, the menu will jump on multiple rows, partially overlapping the header image.
If you’re getting different behaviour between browsers try reducing your menu length with 2-3 characters (as browsers may compute the menu elements / lengths slightly differently).