Firefox for Android Community Forum

Hello,

I'm an older person who's still learning how to use my phone.

I recently noticed a symbol I don't understand. It's on the right edge of the Navigation Bar and is a box with 3 lines (text?). When I click on it, it turns the page black and simplfies what I'm seeing. I've added a couple of images below. It's even on this page I'm using now.

What is this?

Thanks!