Firefox: If you struggle to read small text on websites, but don't want to scale every element on the page to make it legible, No Small Text is a fantastic extension that only affects the text you can't read.
The idea of this extension is simple, but the execution is perfect. In its settings, you simply specify the smallest font size that you're comfortable reading. Then, just toggle it on and any text that's too small to read will be scaled up. What I like about this is that it doesn't affect all the text and images on the page; it only touches the small stuff. Even if you have great eye sight, this would be particularly helpful for reading any fine print on a site's footer.