Do the basic computer clean-ups - make sure all patches are up to date, virus scan, chkdsk, defrag (assuming it's a Windows machine). Clear out the Firefox cache. In fact, I'd be tempted to save bookmarks, then blow away the FF profile and start fresh.
If you are running any Norton or Symantec products, uninstall them. Stick on Avast or AVG for antivirus, and don't turn on any of the other features (like web scanning, email scanning and so on. You're trying to reduce load on your internet connectivity). Obviously, be a bit more cautious when opening email messages while you have email AV scanning off.
If none of that helps, try an alternate browser like Opera, which is usually pretty rock-solid. The aim is to see whether it's your dial-up that's causing the problem, or your security/AV suite.
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If you are running any Norton or Symantec products, uninstall them. Stick on Avast or AVG for antivirus, and don't turn on any of the other features (like web scanning, email scanning and so on. You're trying to reduce load on your internet connectivity). Obviously, be a bit more cautious when opening email messages while you have email AV scanning off.
If none of that helps, try an alternate browser like Opera, which is usually pretty rock-solid. The aim is to see whether it's your dial-up that's causing the problem, or your security/AV suite.