Since firewalls essentially filter data, the answer depends on what kind of filtering you do.
If you filter based on IP address (for example), you can say that your firewall is filtering at layer 3. If you filter specific ports, you can say you're filtering at layer 4. If your firewall inspects specific protocol states or data, you can say it operates at layer 7.
The truth is that most firewalls do all these things in combination. So practically speaking there really is no useful answer to your question.
May I suggest you also read this question about the OSI model.
MAC addresses are layer-2 addresses in the frame header. The layer-2 frame encapsulates the layer-3 IP packet. The layer-2 frame is stripped off at a layer-2/3 boundary (e.g. a router). If the layer-3 packet needs to be forwarded through another layer-2 domain, a new layer-2 frame is created using MAC addresses in the new layer-2 domain to encapsulate the layer-3 packet.
Conceptually:
To the router:
<L2 frame><L3 packet><L4 segment>PAYLOAD DATA</L4 segment></L3 packet></L2 frame>
In the router:
<L3 packet><L4 segment>PAYLOAD DATA</L4 segment></L3 packet>
From the router:
<New L2 frame><L3 packet><L4 segment>PAYLOAD DATA</L4 segment></L3 packet></New L2 frame>
The original layer-2 frame will contain the source MAC address of the sender, and the destination MAC address of the router's interface in that layer-2 domain.
The new layer-2 frame will contain the source MAC address of the router's interface in the new layer-2 domain, and the destination MAC address of the next hop in the new layer-2 domain.
The layer-3 packet will contain the layer-3 source IP address of the originator of the packet, and the layer-3 destination address of the final layer-3 destination.
Layer-2 MAC addresses are removed at each layer2/3 boundary along the routes to be replaced with the MAC addresses of the new source and destination, but the layer-3 IP addresses stay the same along the path (with certain exceptions like NAT).
Best Answer
The role of modems is to modulate and demodulate (hence the word modem) data so that it can flow over the phone line, effectively transforming the data from digital to analog and viceversa. For this reason, they can be considered Layer 1 devices. They cannot work on Layer 3 of the OSI model (routers and Layer 3 switches can do that).
Correct, but they are not necessarily used only with telephone cables. They could also be used with a cable (CATV) line.
Yes, the data flows in the wires to the next network devices. In summary, the next router in the nextwork will receive the stream, deencapsulate it until the layer 3 IP packet inside it is exposed, and will use this information to decide where to send the data. This is repeated until the data arrives at the destination.
And to answer the question in the title: yes, they are still used today. It's just that you don't see them most of they time because they are built-in the routers you buy in the shops.