WebKit makes relatively heavy use of reference counted objects, though a custom reference counting implementation is used rather than std::shared_ptr. That said, there has been an increasing emphasis on preferring unique ownership to shared ownership of objects where possible.

I'm a little confused by your throwaway comment about Gecko being the smallest browser engine though. WebKit weighs in at less than 3 million lines of code, much smaller than the size you quote for Gecko.

Yeah, I was thinking Chromium when I wrote that and should have been more specific. 6M is all of Firefox.

And Gecko uses reference counting a lot too, of course... but most objects are stack allocated or uniquely owned. It's a very different situation from a language in which all objects are reference counted.