Wurdearring: 4 fan de 5 stjerren

Ok I know I'm stupid. Once you download, what do you do then?.Tried everything but nothing happening. Its an XPI file so do I open it, when I double click it asks me what I want to do but not tech savvy, have no idea, thanks

Dizze beoardieling giet oer in eardere ferzje fan de add-on (1.1.1).