I'm using Waterfox, based on pre-57 Firefox, ie without the new system of add-ons.
I've had a look around, there are some add-ons that allow custom style sheets.
At the moment: view>page style, gives two choices; basic page style and no style. No style is useful sometimes when the page doesn't display properly because I have javascript turn off.
Ideally I would to be able to have more choices, eg one where images get resized to a preset maximum.
I'll try some of the add-ons and see what they're like.