In a recent update to Firefox (v46), when running under KDE on Kubuntu 14.04, the choice of text colour when hovering over images or other items in the browser that pop up a tooltip was essentially unreadable. Here's an example:
This is due to Firefox (for whatever reason) using the "WindowText" system colour rather than the "TooltipText" colour.
There are a couple of ways to fix it. The first is to go into the "Colors — KDE Control Module" and select a new colour scheme that has a better contrast in those two colours. Out of the defaults in KDE, I found Honeycomb, Norway, Obsidian Coast, Oxygen Cold and Zion all looked okay.
The other way is to override the tooltip colours.
On the same dialog, click the "Colors" tab, and find the Tooltip Background colour. If it's a dark colour, change it to something light, or light if it starts dark. You might also want to change the Tooltip Text colour to complement it.
Another option is to change the Window Text colour, but this colour is used in many more places, so will affect the look of all other applications.
This thread on the Arch Linux forum has a lengthy discussion on the issue.
Addendum: for Tree Style Tab users, changing the colour settings doesn't help. The selected tab highlights itself using the Window Text colour, the same colour as the text. To get around that one I changed to the Oxygen theme, rather than the default theme. That fixes most things up, but some sites override the colour of the text in dropdown boxes, but don't override the background...so it becomes close to unreadable. Annoying.