btnx is no longer available on newer Linux Mint (probably Ubuntu too) releases. This is a working alternative.
sudo apt-get install easystroke
A new icon appears in notification bar. Open the application. In Preferences->Additional buttons add the buttons you want to configure. I need the thumb scroll to switch between tabs – forward, back and close. So I add the three buttons.
In Actions tab, I mapped the new buttons to their keys (CTRL+Tab, CTRL+Shift+Tab, Ctrl+W). It seems I can not map the buttons as strokes before adding them as additional buttons. The on-screen notifications can be disabled in the Preferences tab.