The reason Ctrl-F4 doesn't work in KDE is because KWin uses the same keyboard shortcut for switching between workspaces. If you would really want to use Ctrl-F4, you might want to consider editing the KWin keyboard settings.
To do that, open System Settings and go to Shortcuts and Gestures.
Then navigate to Global Keyboard Shortcuts and choose Kwin from the KDE component dropdown.
Scroll down all the way until you see Switch to Desktop 4 option. Click on the shortcut and choose Custom and either select a different shortcut or leave it as empty.
You should now be able to use Ctrl-F4 to close tabs in Firefox.
The reason Ctrl-F4 doesn't work in KDE is because KWin uses the same keyboard shortcut for switching between workspaces. If you would really want to use Ctrl-F4, you might want to consider editing the KWin keyboard settings.
To do that, open System Settings and go to Shortcuts and Gestures.
Then navigate to Global Keyboard Shortcuts and choose Kwin from the KDE component dropdown.
Scroll down all the way until you see Switch to Desktop 4 option. Click on the shortcut and choose Custom and either select a different shortcut or leave it as empty.
You should now be able to use Ctrl-F4 to close tabs in Firefox.
Related:
In Xubuntu, keyboard shortcut settings can be find in this path:
Settings→Settings Manager→Window Manager→Keyboard