Apologies - I forgot to include this this:-
Previously I have had the *alternative* file picker thingy (I assume
that's what you meant by "kdialog") working instead of the default
iceweasel one - but the experience was flaky and breaks on upgrades.
NOTE: it *still* uses GTK not QT. Use an Iceweasel theme and KDE QT
configurator to make it "look" like QT.
about:config -> filter for "ui.allow_platform_file_picker"
set the boolean value to "false" (double-click on it)