Sure, take a look at what is in it and unless there's something you want to keep (unlikely), clean it.
Unluckily updating Firefox to current is close to impossible, even if we had a skilled enough team to do it.
As for why Dooble don't work, I've run out of ideas for now.
Do other Qt5 apps work? There's a bunch of examples under D:\usr\lib\qt5\examples, perhaps test some, including under webwidgets. Might need to install aQt5 example package