'Emacs style' delete line shortcut (CTRL/U) doesn't work correctly in Firefox

[Chris Green]

I have the 'Emacs style' keyboard shortcuts enabled on my [x]ubuntu
18.04 systems.  The shortcut I use most often is the (not really
Emacs) CTRL/U for deleting a line of text.

This should delete the whole line of text even if the cursor isn't
placed at the end of the line.  It *does* do this in other GUIs but in
Firefox it just deletes from the cursor to the start of the line.
This makes the shortcut much less useful as it means I need to move
the cursor to delete the line and this means I have to use the touchpad
or mouse.  The whole point (for me) of CTRL/U is that I don't need to
leave the keyboard to enter a new line.

I'm fairly sure this *used* to work in Firefox.  Is it therefore a bug
I can report? .... or is there a workaround, i.e. a simple, keyboard
only, 'delete line' shortcut?

-- 
Chris Green