How to fix grey menu blob

Using Firefox 40.0 on Fedora 22. Using KDE. I've adjusted the layout.css.devPixelsPerPx setting since my screen is on a Google Chromebook Pixel 2015, and it's adjusted to 1.5 . Anything above 1.0 on that setting causes this issue.

Issue in detail is that clicking on the menu in the top right corner produces a grey 'blob' to occupy almost half of the menu - hiding the actual menu behind it. Screenshot attached.