Firefox let's you set custom zoom levels in about:config, to any percentages, e.g. .6, .65,.7,.75,.8,.9,1,1.05, … etc.
It's very useful, especially when Chrome/Firefox plugins get deleted from Webstore/Addon store for giving this functionality.
This is extremely useful when you use your browser to read online comics/manga/ viewing image archives - and for many other things.
This is also pretty crucial feature for good Accessibility.