I use KDE (Kubuntu 14.04), the Oxygen theme and Firefox, and FF46 has several bugs with GTK3, the most annoying being displaying tooltips as dark text over a barely darker background.
So I'am looking for a temporary change in them settings to either have the text in white (but I think the problem is that FF46 isn't using the theme color) or having a lighter background.
As far as I can't tell the tooltip background is not even a solid color but a vertical gradient, from roughly 60,53,48 at the top to 42,37,33 at the bottom and I cannot relate these to some contents of the theme in /usr/share/themes/oxygen-gtk/gtk-3.0/ (kdeglobals, oxygenrc or the two CSS files).
So, where are these colors defined?
You can change the colour in the System Settings, which will fix it for all applications, not just Firefox.
On Kubuntu 14.04 LTS: System Settings > Application Appearance > Colours (side selection) > Colours (the tab) > Tooltip Background and Tooltip Text