I also use Ubuntu 17.04, 64 bits, with a HP Compac laptop and needed to exchange CONTRL and CAPS-LOCK keys, as recommended for use with Emacs. Since I've seen many posts about xmodmap not working, I'll contribute my 2 cents, even though the remapping I did is not exactly the one you intend.
Following the advise in the man page of xmodmap, I invoked this script from the terminal (I called it 'remapkeys' and put it in home directory):
remove Lock = Caps_Lock
remove Control = Control_L
keysym Control_L = Caps_Lock
keysym Caps_Lock = Control_L
add Lock = Caps_Lock
add Control = Control_L
(without spaces in between)
xmodmap -verbose ~/remapkeys
Everything worked as intended, even if Emacs is already running when I do it. CONTROL key and CAPS-LOCK keys are exchanged.
The same procedure should reverse the changes, but you can always re-start and the keymap turns back to normal.
So in my case, xmodmap worked right but the parameters (in their correct order) are tricky, and the the man page is rather obscure for a beginner.
Best Answer
To change the Backspace mapping in Firefox to the same as in Windows:
about:configin Firefox addressbar and clickI accept the risk!browser.backspace_action0Reference: Browser.backspace_action