1. It is usual to only see a URL without a port assigned to it as it should normally use the default http port (80). When a port is assigned it is usually shown at the end of the URL as Colon, Port number (e.g. :25), in this case the error message text.
2. The reason I find it strange is that is an email port, used for sending email, not browsing the web. Since firefox isn't an email client but a browser, no port should have needed to be assigned it would have used port 80.
3. When ports are used for different reasons to the standards it makes me a little suspicious of the reason why it was done, usually to avoid something (like a virus scanner monitoring the default ports).
4. The avast Internet Mail scanner monitors port 25/110 and is expecting to see data using the SMTP/POP3 protocols, if it doesn't find this it may take longer to inspect the traffic because of this (speculation on my part). This may lead to a period of inactivity which is incorrectly identified as an "internet connection timeout".