I have a WIFI and a LAN both connected to internet (with 2 different provider).
How can I set my browser to use 1 instead of the other to browser?
This is not normally done on a per-browser basis, but rather is done system-wide using routing metrics:
The route with the lower metric for a given connection will be chosen. Windows by default will set a LAN connection to have a lower metric than a wireless connection, but there are things that can cause this to change over time. You can set the metric for a given route using the command line program aptly named