Hi,
I'm in Ubuntu 16.04 and Firefox 53.0 64 bits I have this behavior:
As you can see, the mouse buttons are inverted (only in Firefox). Left click goes to the mouse position.
How can I reverse this behavior? (This didn't work) Thanks in advance!
I’ll response to myself.
It’s a GTK3 behavior 
then you have to custom the theme, in Ubuntu MATE, for the default theme, create this file:
costales@desktop:/$ cat /usr/share/themes/Ambiant-MATE/gtk-3.0/settings.ini
[Settings]
gtk-primary-button-warps-slider = false
costales@desktop:/$
Best regards.
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