Comment # 5 on bug 1022830 from Felix Miata

(In reply to Guillaume Lang from comment #2)
> PARTIAL WORK-AROUND
> In '~/.Xresources':
> Xft.dpi:    192

The problem exists because libgtk >3.16 forces Xft.dpi to 96 if null, because
of the commit referred to in both comment 1 bugs. KDE's "Force fonts DPI:" in
systemsettings does the forcing by setting Xft.dpi, so there is a workaround
that doesn't require changing .Xresources, but either workaround must be
manually updated and session restarted with every display change instead of
having the server's DPI automatically used. KDE3 and TDE also force DPI by
setting Xft.dpi non-null.

Mouse pointer here gets larger over GTK3 Mozillas than it is over other
windows.