I have continued experimenting regarding this problem and came up with the following:
I used the Synaptic Package Manager to do the following updates:
linux-firmware (1.79) to 1.79.1
xserver-xorg-input-evdev (1:2.7.0-0ubuntu1) to 1:2.7.0-0ubuntu1.2
xserver-xorg-input-synaptics (188.8.131.522-0ubuntu5) to 1.6.2-1ubuntu1~precise2
xserver-xorg-input-vmmouse (1:12.8.0-1) to 1:12.9.0-0ubuntu0.1
xserver-xorg-video-intel (2:2.17.0-1ubuntu4) to 2:2.17.0-1ubuntu4.1
xserver-xorg-video-vmware (1:12.0.1-1ubuntu1) to 1:12.0.1-1ubuntu1.1
Some of them were most likely not connected to this problem and I only updated them for completeness' sake. But I think (correct me if I am wrong) that both the linux-firmware and the xserver-xorg-video-intel update could have been connected to it, since when I started Firefox again, the sound problem in the youtube videos was gone! So this is a success. Unfortunately, it is only a partial success, since the videos have now started to become slower when I maximise them. I have not checked it yet, but maybe the forum has some threads on this new problem. I will see about this soon. (Feels a bit like talking to myself, but then again maybe that helps someone else.).
EDIT: Spoke too soon! When I restarted the laptop, the videos started stuttering again. So it's back to the beginning.
EDIT 2: Ok, two days ago I was bored and installed Battle for Wesnoth" on my laptop. Pretty nice game, but apart from being fun, it also showed me that my laptop does not only have sound issues with youtube. It also seems to struggle with the game sound (it works, but occasionally has a disturbance or so). I have come to the conclusion, then, that the problem is not necessarily the flashplayer, but rather something to do with the sound driver. I have had a look at a thread on replacing PulseAudio with ALSA (in this forum: viewtopic.php?f=42&t=70675&p=410093&hilit=alternative+to+alsa#p410093
) and tried to find more information on my hardware (as I said, I am pretty new to linux and not that experienced with some hardware issues, so I found it hard to believe that my sound system is just called "Intel HD Audio Chipset". But hey, it is!). This guy http://www.prolixium.com/linux-t60
had a nice introduction to Linux on a Lenovo T60 and I believe that, although mine is a slightly different one, the statement that the Intel Audio Chipset mentioned above works fine with ALSA might also be relevant to my laptop. I checked the synaptic package manager and it looks like my laptop is using both PulseAudio and ALSA packages. Before I start tinkering with de- and re-installing some of them: is it advisable to completely clear either of the two from the system? Or is there something I don't know yet on how they relate to each other in some way?
Many thanks in advance!