Contains Spoiler:

I've found so many clues in the game to be able to understand the story, I got the animation with the cigarette and the one from the silverfish. I know that I have to use the cigarette to activate the smoke detector etc... but the damn cigarette in the ash tray above the pissoir hasn't burnt down, it's still too big to move!
What do I have to do to make it small enough to move it?

If the cigarette is still placed in the ash tray, you will not be able to move it. You have missed something along the way. And, you will have to explore to find and complete any action you have missed.

That's what I've read after googling, but what exactly do I have to do?
See every animation in the game? I thought I did.

If you haven't been able to unlock the ability to drop the cigarette down, then you haven't done everything you need to do. And you most certainly haven't watched *every* animation in the game, there are loads more after that part of the game. Look for a good walkthrough, preferably one without spoilers, since I haven't played the game for quite some time and I would just resort to looking at a walkthrough to tell you what you need to do, anyway.

EDIT: check this one if you're stuck, but try not to spoil too much: http://www.gameboomers.com/wtcheats/pcBb/badmojo.html

Maybe I should have added, that I'm already through the game, I've made a save shortly before the timer starts though.
I just don't get what I missed ingame to trigger this at the end.