OK, some update.
I kept watching Task Manager, and I've noticed, that one of
svchost.exe processes CPU usage kept jumping between 0 to 40%.
I investigated further, using Process Explorer, and that particular svchost process was responsible for 4 other sub-processes.
Two of them well definitely needed by Windows, so I didn't touch them, but two others were worth to play with them.
At first, I disabled
Telephony service, but it didn't have any influence on svchost jumping CPU usage.
Secondly, I disabled
Network Location Awareness service, and svchost CPU usage dived to flat ZERO, and stayed there.
Now, interesting part - Firefox connection.
Actually, I have no clue, as of now, what's the connection between disabling Network Location Awareness service, and Firefox RAM usage is, but...
As I stated before, when I start FF3, it's RAM usage is at around 80K, but it pretty rapidly grows to 200K, and after 1-2 hours of usage, it gets to 300-400K, and at that point FF becomes extremely sluggish. Even, if I'm not doing anything, but watching Task Manager, I can see FF RAM usage growing in front of my eyes. I'm forced to restart.
Now, after disabling Network Location Awareness service, FF has been open for about 30 minutes, and its RAM usage stays at around 115K. I'll see in another hour, but so far, so good.
Also, when I shut down FF after disabling Network Location Awareness service,
firefox.exe service disappeared from Task Manager almost instantly.
This is what BlackViper site has to say about Network Location Awareness service:
Collects and stores configuration information for the network and notifies programs when this information is modified. If this service is stopped, configuration information might be unavailable. If this service is disabled, any services that explicitly depend on it will fail to start.
It has been reported to me that Real Player may require this service, otherwise it will stall. It has also been reported to me that Windows Live OneCare firewall requires this service.
So far, I didn't notice any negative impact by disabling the above service.