No, but it is entirely possible to build desktop-installable apps using the Firefox internals, with XUL as the primary interface technology. This was the idea behind XULRunner, a Firefox runtime package for you to build your apps with.
The XML-based XUL isn't terrible, either, considering the respectable set of widgets and the ability to control all the styles with CSS. The nasty part, in my experience, was trying to get through the messy FFI that was XPCOM.
https://developer.mozilla.org/en-US/docs/XULRunner
The XML-based XUL isn't terrible, either, considering the respectable set of widgets and the ability to control all the styles with CSS. The nasty part, in my experience, was trying to get through the messy FFI that was XPCOM.