Note: This isn't my solution, all credit goes to "nickbeee" who posted a couple of lines on the firefox site. I've had to document this for our internal users and figured that other people may benefit from a more detailed explanation.
Also: I'm a stickler for long explanations with pictures....
First: The problem that you see looks like this;
It states on the webmail screen simply (and cryptically)
A problem has occurred which may have caused the current operation to fail.

To get it, browse to:
and click Add to Firefox.

You'll be prompted to install the extension and then you'll be told to restart Firefox.

Note: To get to the menu in Firefox, press the Alt Key once.
Click Tools, then Remote XUL Manager.

A mostly empty dialog box will appear.
Click Add.

A text input box will appear. Type your mail domain in here and click Ok.
If you use several domains, you may want to add the others in now.
