Links in E-Mail Program Sometimes Don't Open in Firefox

I use Eudora and Firefox. If I click on an e-mail link *and* the site
is already open, no problem. The problem occurs when the site
has not already been opened in the browser. What then happens
is a very long wait followed by *IE* opening with "about:blank." If
I am patient enough, the correct page will finally open in Firefox but
it can take a matter of minutes.
I think this is going to be hard to diagnose. I have set Firefox
to open every extension and protocol, and have set it as the
default browser, but this still happens.
Any ideas?
Thank you.
Frank