Hans-Karl,
The kneeling soldiers also could fire. It was a usual practice in the 18th century: when an infantry unit was ordered to open fire, the first rank knelt, the second and third stepped up to the first rank as closer as possible, in the same time the soldiers of the third rank shifted slightly to the right to be able to level their muskets between the heads of the men of the second rank. Against enemy infantry, all three ranks fired either at once (volley fire), or one by one (fire by ranks). Against enemy cavalry, the first rank usually did not fire together with the second and third ranks, the muskets of the first rank were kept loaded to the most critical situation.

After the Seven Years War, there was a hot discussion on the kneeling of the first rank, but even some of those officers, who argued that the kneeling had to be abolished, believed that it was useful against enemy cavalry, for example, Prussian officer Hans Ehrenfried von Diebitsch und Narden, who joined the Russian army in 1798:
Diebitsch H. von, Gedanken über und von dem Soldaten. St. Petersburg, 1801, 2 vols.

Best wishes,
Alexander