The width of the menu in "More tools..." popup menu is broken on Firefox

[SolsticeSpectrum]

Even tho the addon successfully expands the popup width, the inner width is limited by .PanelUI-subView[extension="true"] class

But when I manually rewrite this class's min-width, another issue comes in.

I set the min-width to 500px and even tho this is way over 500px the there is extra space. I suspect that this extra space is calculated from the original width of the menu + these 500 pixels. I don't know.