If they liked working on Qt, I wonder why they didn't consider porting Sailfish Browser - https://github.com/sailfishos/sailfish-browser - based on Gecko?
I think they do, just an old one as ESR AFAIK does not guarantee you stability of the internal APIS needed for embedding between ESR versions.
I think it's at least 45 ESR, as mentioned in release notes ~year ago: https://blog.jolla.com/hossa/
There is an issue tracking the Gecko update on the community discussion/bug reporting tool used for Sailfish OS: https://together.jolla.com/question/133621/update-gecko-in-t...
The source of the Sailfish OS browser lives here: https://github.com/sailfishos/sailfish-browser
(Even though Sailfish OS is sadly not fully open source, the browser is as are other core applications and frameworks.)
The official browser for SailfishOS is Gecko-based. The SailfishOS browser source code is FOSS, so we can see for ourselves, from https://github.com/sailfishos/sailfish-browser :
> Sailfish Browser uses Sailfish Silica Qt components for the browser chrome and gecko engine with embedlite Qt5 binding.