If you go into Gmail's General Settings screen, you can uncheck "Open web links in Gmail". All the links I open in Gmail open directly in Firefox for me after doing that.
If I leave that box checked, it does use a web view that is "Powered by Firefox", but I always prefer to have it just open a new tab in my regular Firefox app.
If I leave that box checked, it does use a web view that is "Powered by Firefox", but I always prefer to have it just open a new tab in my regular Firefox app.