Why is my window falling down?

It sounds like you're having issues with your tilt latch. These latches are what keep your balance upright in the track; when you press the thumb levers in, you can tilt the window in for cleaning.

Unfortunately when these latches break, that means there's nothing there to keep the window from tilting in. Is this the problem you're experiencing? If I've misjudged the situation, let me know and we can pursue other options.

Assuming that I'm on the right track, the bad news is that I don't recognize your latch based on these photos. The window looks like it might be a little old so there is a chance we don't sell a good replacement. Before I say anything for sure, I'd like to see a photo of the latch removed from the window. That way I can see the whole thing without obstruction and would be better able to check our inventory for a match.