I notice on the website on some I checked a font-weight: 500 setting (likely bolder). In some cases this might not be sufficient (normal bold is 800).
What font is used if you check this on the Font tab in the right panel in the Inspector?
You can right-click and select "Inspect Element" to open the builtin Inspector with this element selected.
You can check the font used for selected text in the Font tab in the right pane of the Inspector.
Note that this is not about the font-family CSS rule, but about the font that Firefox actually uses as shown in the Font tab