How do I tell Picard which browser to use?

On Windows, macOS, GNOME and KDE, Picard uses the default browser that has been configured for the system. On other systems, you can use the BROWSER environment variable.

For example:

export BROWSER="firefox '%s' &"

Another approach that works in some GNU/Linux systems is the following command:

sudo update-alternatives --config x-www-browser

This should present you with a list of existing browsers in your system, allowing you to select the one to use by default.