Firefox

Well, user agent switch, will switch your user agent, but that is not the answer ponly feature of chameleon. It sets your referer pcolicy, cookie policy, spoofs some data and restricts certain APIs etc. Other addons I use as well would be audio equalizer, which allows me to change volume more precisely and ofc eq my sound. To that also screwmycode.in which changes the pitch together with speed, so you can make nightcore music easily and eq some bass back into it with the previous addon.

Nobody can forget amazing conteiners addons, like temporary containers, multi account containers and facebook container. They can revolutionise your workflow.

Other then addons, you can also set up profiles, to which you can create shortcuts, which is cumbersome, but works really well.

Also, firefox has a nice amount of CSS themes to apply onto it, making it look like a completely different browser.