The addon is now working in Chromium browsers.
The code is more simple and more robust.

- Changes to keymap and deleted keymaps
Shift + ss and Shift + xx keymap has changed.
Now:
Shift + ss : change current tab to https://duckduckgo.com
Shift + xx : change current tab to https://google.com

Before Shift + ss opened duckduckgo.com in a new tab and Shift + xx opened
google.com in a new tab. But the code to that dosn't work anymore. Now that code opens a new window.
To open duckduckgo.com with a shortcut you can install
"New tab with duckduckgo.com".
https://addons.mozilla.org/en-GB/firefox/addon/new-tab-with-duckduckgocom-ext/

Shift + tt keymap is deleted. Before Shift + tt opened a new tab. But the code
to that dosn't work anymore. Now it opens a new window.

Shift + dd keymap is deleted. It worked with Shift + tt and the old Shift + ss
and the old Shift + xx.

- If page is duckduckgo.com then a search field is focused. Therefore we don't
want to enable to be able to scroll with j, k, g and G. To do that click
outside of the input field.

- Some blank space has been deleted.

- Some links about how to program tabs has been added.

Changed shortcuts:
window.history.back() is now Shift + hh
window.history.forward() is now Shift + ll
open new tab is now Shift + tt
open duckduckgo.com is now Shift + ss
open google.com is now Shift + xx
close tab is now Shift + dd

time to check if a key is tapped twice is now 0.5 second.
open new tab is now tt
open duckduckgo.com is now ss
open google.com is now xx
window.history.back() is now hh
window.history.forward() is now ll

Shortcut for open google.com in new tab. Shortcut is:
ctrl + shift + x

Two shortcuts has changed:
shift + t is now shift + ctrl + t
shift + s is now shift + ctrl + t

Apache License, Version 2.0 added.