Looking at this page title with the background image, in FF (not private mode) there’s a gap whereas in Chrome (not Incognito mode) the heading looks centered.
If I was to look at the same page in FF in Incognito Mode, the page title looks fine. I’m not sure which view to rely on.
Do you use any reset styles to zero out all the default browser padding and margins and start from scratch with your own styles? That way you will avoid the browser differences.
Still looks the same to me in Firefox and private version.
I would however get rid of the before and after rules on masthead because a flex container doesn’t need containment and the items may get rendered as flex items.
Your picture looks like you are losing the 25px padding-top on header.site-header some how.
If I adjust the padding to 97px to .site-content, then it looks normal in Firefox standard, but in Chrome and IE you can see a slight imbalance in the page heading and that doesn’t necessarily fix the problem.
I’m still not seeing the issue but you should not be messing about with padding and content height to make room.
You have a fixed header so in order to have content underneath you need a known fixed height. Set the height of the header in pixels and vertically centre the inner content without the need for padding (you are using flex anyway). Then you can start your site content at the exact padding px distance that you made for the height of the fixed header,
Generally though I avoid fixed headers for this reason and I find a better alternative is to have a normal header but once the user starts scrolling then you can fix it in position (using js). In this way you avoid the content being underneath the header except when you have scrolled.
FYI, there is a WordPress plugin that hides the admin bar while you are viewing the site while developing, which you can toggle on and off. I often use it to get rid of the admin bar. Try looking for “Admin Bar Toggle” in the Plugin repository.