There are some problems using Firefox in Kubuntu.
- The menus are ugly and do not use the KDE style.
- The file browser for uploading or opening files is also ugly, and is not the default KDE mini-file-browser.
- The filetype icons are not shown correctly in the Downloads window.
- In the small Downloads window, if I right-click a file and select "Open Containing Folder", nothing happens.
gtk3-engines-oxygen is installed and "oxygen-gtk" is the default GTK style in System Settings > Application Appearance > GTK.
Here's an example of what it looks like: