1. Bandwidth measurement.
Open "Tools" menu and click the item "Bandwidth Meter". You can perform upload and download bandwidth measurement. It also shows your public IP address and public domain name.
2. Broadband troubleshooting.
When Firefox fails to load a webpage, the extension will automatically check whether Internet conennction is working or not. You can perform further diagnostic tests (ping, traceroute) or check the archived version of the failed webpage. Just try to type a non-existent domain name(e.g., qwqwqwq121212.com) to see this in action ...
3. Perform diagnostic tests on your PC
Use the diagnostic and configuration buttons to check connectivity with NIC card, local gateway, and websites and to display local IP configurations (IP address, MAC address, default gateway, DNS server, DHCP server etc).
1) This is a Window version of the extension. If you use Unix or MAC, please see https://addons.mozilla.org/en-US/firefox/addons/versions/2450 (pending review)
2) For FF3 users, please use version 1.2.5 (under review): https://addons.mozilla.org/en-US/firefox/addons/versions/2360 - it fixed all bugs for FF3.