You can configure it by going to Tools > Options... > Network and Internet Options > Web Browser Setup.
Unlike Windows and MacOS, Linux has not traditionally had a default web browser OS setting, other than Firefox, well, before that really Mozilla, being a defacto default web browser.
Now, some Linux distributions (inconsistently) honor the $BROWSER environment variable. Some use xdg-settings. Some use update-alternatives. Some just fall back on MIME type settings for text/html.
Anyway, I'll file a feature request to update our default web browser selection code for Linux.