I recently upgraded to 64-bit version of Kubuntu and since then Firefox lost most of its icons. From the screenshot you can see that some of the menu icons are gone, Firefox window doesn't have an icon, tabs have no icon for close button, new tab button doesn't have its icon. Obviously, it wasn't a great experience using the browser in this state.
Turned out it was because I was running an incompatible version of Firefox on my machine. I always download Firefox from Mozilla's site as a tarball and run it from the command line. (I do that because Ubuntu historically has been very slow in updating the officially supported Firefox version.) The Firefox tarball I had was a 32-bit version.
The solution? Upgrading to 64-bit Firefox, obviously. But Mozilla, for some reason, doesn't distribute 64-bit version of its browser. I have started using the Ubuntu-supplied Firefox now and everything is back to normal.