The firewall interface is Gigabit capable and set by default to operate at 1000Mbps, but cannot go faster than 100Mbps. This could be because the ISP router is only set to run at 100Mbps, or you are using a Cat3 cable instead of a Cat5e or Cat6 cable; Cat3 can only link at 100Mbps.
Run command netstat -ni, if you are taking RX-ERRs you almost certainly have a duplex mismatch with your ISP running at 100/half and your firewall running at 100/full. Assuming you are supposed to be running at only 100Mbps due to your agreement with the ISP, tell the ISP to hardcode to 100/full and hardcode your firewall interface to 100/full as well. Duplex negotiation is more or less broken in Fast Ethernet (100Mbps) but was thankfully fixed with Gigabit Ethernet.
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