And yes, this may be an official bug from… 2004 but I stumbled upon it just yesterday!
There is no way a front-end developer can be aware of each and every one of the confirmed bugs of every browser — we only know the most important or relatively new ones. Anyway, you should be aware that Firefox does not support any value of the
visible which is the default (example). If you need to hide the contents of a
<fieldset> you need to wrap them inside a