On my iPhone I have one called Save All Images. Basically you know how lots of sites especially any kind of social media where they want you there to see things… anyway they use a script or whatnot to prevent holding down on an image to get the save photo prompt. This is an extension that loads all images that are single level referenced and you can save any/all. I use it almost daily. Instagram, and the like, especially, they hate the notion of someone saving something.
In Firefox on my computer I have Element Blocker. Some websites just waste a lot of screen space so if you’re browsing a ton of content you’d like to get the most room, but some stuff takes up portions of the screen fixed and not movable. This extension lets you select an element and it makes it just disappear. It is absolutely essential.