Enlarge icon to 48x48px
Based on "tab new" icon form Tango library
https://www.iconfinder.com/icons/118799/new_tab_icon#size=48
+ resized chrome://browser/skin/Privacy-48.png
Add icon source
Used tab icon from Tango library
http://tango.freedesktop.org/
https://www.iconfinder.com/icons/118799/new_tab_icon
and mask icon from built-in Firefox theme
chrome://browser/skin/Privacy-16.png
chrome://browser/skin/Privacy-32.png
chrome://browser/skin/Privacy-48.png

Note: there is also 64x64 icon, but tab icon is too blurred...