It's a good sign that you only noticed this on FFR... as it means you come here more than any other site.
Other than View » Zoom, this could also be caused by using custom (or incorrect) display settings.
These "lines" are 1 pixel wide. It is not possible to make this smaller, if this option is chosen. In this case, random "lines" may disappear. Unless it is magnified by a multiple of 100%, this will cause apparently random "line" widths.
("line" is in inverted commas, because in some places, this is a line, and in other places it is the absence of another element.)
There is also a feature, which I think is unique to Firefox, and this is to zoom only the text. All settings are available from View » Zoom.