Personally, I have it set as my default browser, so that any links that I click on in other applications open up in it and then I also have my full-blown Firefox installed for when I need to do actual browsing.

Works out really well, as Focus starts up extremely quickly (uses the native Android Webview), and I usually don't need each and every link in my browsing history, and it also means that my proper Firefox doesn't get filled up with random tabs that I didn't close out of properly.