Mohtaram memad Sahib,
Now I got your point. I hope the following explanation will suffice:
The altitude at which ADS-B (automatic dependent surveillance - broadcast) can detect aircrafts is limited by the types of receivers and geography. In some parts of Europe, we can detect aircrafts through ADS-B flying as low as 500 feet. In Malaysia there are only two ADS-B receivers and with a limitation of 30,000 feet. Any aircraft flying below 30,000 feet can't be detected by ADS-B technology. Mind you, flightradar24.com has its own independent ADS-B system in Malaysia.
Your question: How does flightradar24.com detect aircrafts flying below 30,000 feet in Malaysia?
Answer: This is done by SSR (Secondary Surveillance Radar) technology used by Malaysian ATC (Air Traffic Control). Data of aircrafts flying below 30,000 feet is shared by Malaysian ATC with flightradar24.com. That's how flightradar24.com gets information about aircrafts flying below 30,000 feet. Above that, flightradar24.com gets information through both sources, SSR and ADS-B.
Flight data of MH 370: In the case of Malaysian Airlines MH 370, the Boeing 777-200 was equipped with two transponders, one for SSR and one for ADS-B. Both were deliberately turned off. Had the transponder only for SSR were turned off and the one for ADS-B been running, Malaysian ATC would have been blinded for the flight data of MH 370 but flightradar24.com would still have had been able to see it.
I hope the above explanation was able to explain the point.