I tried all the current versions of Firefox in Safe Mode with ETP off and had the same issue. If you refresh the page the 3rd column flashes on the page. Downloaded an older version and I see the 3 columns. see screenshot File a bug and see what Dev says.

Thank you! Do you have the link or form where I can go to contact the developer?

I just noticed in a current version of Firefox that making the page more narrow it shows 3 columns.

 see screenshot

Who's problem is it now... I vote site dev team, but I'm not dev and just guessing.  ;-)

While resizing the window, you'll see the 3rd column moving around until it moves into place.  ;-)

Yes! I see exactly what you mean ~ I am just not sure how to contact a DEV to report this :)

https://bugzilla.mozilla.org

You usually get this when there are rounding errors and it doesn't fit if you use width: 100% / 3 like in your style rules (i.e. 100%/3 * 3 > 100%). Possible workarounds are allowing for 1px rounding margin: width:calc(100% / 3 - 1px);

.one-third,
.span_1_of_3 {
 width:calc(1009% / 3 - 1px);
}

So in your opinion is this a bug or site dev team or both.  ;-))

This is likely about how the browser rounds (100% / 3). It will work if the browser never rounds up (i.e. truncates or rounds down), but fails if the browser rounds x.25 up to the nearest 0.1px (or 0.05px). If it doesn't fit like when ((100% / 3) * 3) is greater than 100% then content will be wrapped to the next row.