If you're pressing LMB (mouse1) to throw, press RMB (mouse2) to cook it (you'll hear the sound of the safety pin coming off, an animation will only play if you press LMB and RMB together quickly, see the preview section).

if you're pressing RMB (mouse2) to throw, do the opposite and press LMB (mouse1).

Got that sorted? Great, now to learn the fuse times.

I know this is probably already common sense, but these two despite being incendiary grenades are actually quite different from one another.

To put it shortly :

(Example of the area of effect when you impact a molotov on a roof / ceiling)

(Another example indoors, the ceiling was high enough that only the floor area directly under the point of impact had the incendiary effect)

Credit to @zoya for telling me about this
https://youtu.be/qU1b1nukeOI