Keyboard mapping for HP Laptop’s Email Button

So my laptop is a HP Pavilion DV6 from 2011 or so. It has a row of “quick buttons” on the left, allowing users to quickly open Mail (mailto: in Windows), Browser, Print and even the Calculator.

Recently the Email button is randomly “pressed” on its own, opening whatever that is the default email app and disrupting my works. I know this is the problem because I just “de-associate” the button in Windows’ regedit, and it stopped happening there. In my Ubuntu installation, Thunderbird is opening on its own and I don’t know how to “de-associate” this button.

The version is 19.04. I look forward to your help.