Thanks for the comments, everyone!
As already indicated, openSUSE's Firefox is patched to use KDE dialogs via kmozillahelper.
OK, so the KDE file dialogs are already used automatically for everyone who has the firefox package. Thus, no need to mess around with environment variables.
kmozillahelper supplements packageand(MozillaFirefox:plasma5-desktop), i.e. it should get installed automatically if MozillaFirefox *and* plasma5-desktop is installed (unless you disable installation of recommended packages).
AFAICS, the only problem left is that the kmozillahelper and the thunderbird packages have no dependency relation. To fix this, I submitted a request [1] to automatically install kmozillahelper when MozillaThunderbird and plasma5-desktop are installed. That should help people who have the thunderbird package installed, but not the firefox package. And now I took so long writing this mail that the request was already accepted :-) Best, Tom [1] https://build.opensuse.org/request/show/821566 -- To unsubscribe, e-mail: opensuse-kde+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse-kde+owner@opensuse.org