> As far as I remember, Chrome has used the STL earlier. But I looked now and I see this:

The WTF (Web Template Framework) is actually inherited from WebKit and used in the core rendering engine. I'm not sure how widely it's used in the Google parts of Chromium.

> Gecko also seem to have a lot of custom stuff

That's the old XPCOM stuff. While those string types continue to get a lot of use, the closest analogue to the STL is MFBT: https://github.com/mozilla/gecko-dev/tree/master/mfbt