So which app exactly do you mean you are in when something comes up?
You mean you're in Google Now, looking at the various cards, and you click one of the "recommended reading"-type cards based on your search history and interests, and a dialogue pops up asking which browser to open the link in?
If so, and you have Firefox installed, just check Firefox and then hit "Always" at the bottom of the dialogue. Yfou can later reset it to ask again or choose a different app by going into the Settings --> Apps --> *specific app here* --> Clear Defaults