Each Saturday at 10PM during the summer, a contestant from one of a number of countries puts on a fireworks show over the St. Lawrence River. This Saturday's (August 8th's) contestant is from South Africa.
I found a fan site which describes good viewing places, if you don't want to pay to go to the Six Flags: La Ronde amusement park to sit in special seating. (We did this one year and it was fun, but mainly because bedfull_o_books loves rollercoasters and spent the day riding them.)
Their "Where to view" site is at http://www.montreal-fireworks.com/pyro_map.html, and includes useful maps.
Get there early. We didn't, a couple of weeks ago, and settled for an ok view from some distance off.