I have Filseclab firewall, which I find very simple to use, yet highly customisable. Recently I looked up its "Monitor" showing all the traffic in real time, and I was surprised to see that, seemingly, the firewall itself constantly attempts some connection which may look as follows:

Not really because all the connections are local to the network (the 192.168 range is a private ip range), possibly even local to your machine (do a ipconfig to find out if your machine has the ip of 192.168.1.9). It is possible that the firewall uses the IP protocol to communicate between its components.

Yes; you are right: this is my machine's IP. Still, I don't think it was happening before

That report doesn't look right - it reports the "Local Address" (which should be your PC) as 192.168.1.255 which is not a valid computer address, it is reserved for broadcast use (i.e. sending a message to every computer on the 192.168.1.x network). I'd suggest you query this with Filseclab since it seems to be a bug.

Port 137 is part of the NetBIOS protocol, used for Windows file and printer sharing - so Windows itself is sending out such packets, rather than the firewall. If you do not share files or printers on your network, you can disable NetBIOS which should prevent these reports in future - GKWeb's WWDC being one easy way of doing this.

Thanks for the contribution. This WWDC looked like an interesting software, so I downloaded it. I do not share files or printers on my network, so I disabled NetBIOS as you suggest (+ I also disabled everything else as suggested in the interface). I have restarted my laptop twice; however the interface still shows "NetBIOS will be DISABLED after the next REBOOT". And the same traffic still occurs. Any ideas on what I should do next?

Check that you have disabled NetBIOS over TCP/IP as detailed here.

Great reference advice; thanks! That did the trick.