I'm a former Gnome user and since I tried KDE I grew to really like it, although I have to face more issues/bugs than I would have expected.
One thing that I find really hard to understand is GTK integration. This is what I see when running systemsettings as a normal user:
and this is for kdesu systemsettings (root):
And yet I'm using the same qtcurve, same color scheme, using "numix frost light" for both gtk2 & gtk3, same fonts... I even tried to symlink .gtkrc-2.0-kde4 and .gtkrc-2.0 in root to /home/user/.gtkrc-2.0-kde4
This is firefox (non-active menu items are invisible):
This is opera (menu items are now blue):
And this is leafpad:
where menus look ok, but when I run it from root it has cyan menu items...
The first two screenshots have no relation to gtk, the icon are not related to the visual style. If you want other icons for root, run "kdesu kcmshell4 icons" and pick your preferred icon theme for root.
Neither firefox nor opera are gtk+ applications. Opera will try to make use of the current Qt style (QtCurve), but this has caveats and many GUI style detect opera and make slight modifications to the look to ensure things work.
Firefox uses XUL, which invokes gtk+ for styling.
Primarily to this, both applications use the configured custom theme.
You did not provide the most important information (actual gtk+ and gtk+ integration configuration).
Sorry, the font rasterizing difference was too easy to miss on the preview. It's hinting related (for sure)
I could not find a "numix frost light", but installed "numix" gtk+ themes and spotted that the menubar is inverted - and that doesn't work wit firefox. That's not KDE related, though. You may be able to impact the colors FF uses through some "chrome" css, but I don't know how (you'll have to ask on a FF support forum)