First, kill the mold with bleach and wipe down the whole area to try to get rid of as many spores as possible.
Next, identify and treat the root cause. Mold at the bottom of the inside would suggest that interior condensation is pooling there. Window condensation is caused by two factors:
Interior glass temperatures below the dew point
Humid enough interior air that there's enough moisture to condense on your cold windows
This suggests two potential solutions: decrease the interior humidity level, or get better windows that aren't as cold inside. Getting another dehumidifier or a bigger one is likely to be much cheaper than replacing all your windows with expensive, high-performance triple-pane units which are what you really need to stop condensation in the cold Canadian climate. Window technology has come a long way in the last 15-19 years, but it doesn't come cheap. I'd go with the dehumidifier now if the windows are otherwise in good condition, and start saving up for those better triple-pane windows. You'll feel the difference, and it will stop your condensation problems, too.