Whilst I like Ubuntu as an operating system and generally am fine with Firefox, could anyone explain why I sometimes get errors in trying to get through to various web pages. I don't get the same errors in IE8.

As an example, my telephone billing provider; I can login on both IE8 and Firefox and get into all the screens in IE8 but not always in Firefox as happened to me 30 mins ago (I think the version installed is 3.5 on Ubuntu 9.10). There is a message at the bottom of the screen showing that Firefox is trying to connect to a sub page, but it just can't display.

Is there anything that I can do to resolve this and is it an encryption issue? This isn't the first time I've had issues with Firefox. In Ubuntu 9.04 the screen used to sometimes go fuzzy when I used the firefox version there.

I can think of a couple things that might be going on...
1) some web sites are still designed to work with Internet Explorer. If a web site wants to use ActiveX, for example, this would make it difficult, if not impossible, to use Firefox on Linux. Download and install the User Agent Switcher add-on, select IE, and see if that helps (although it couldn't do anything about ActiveX if that's the issue).
2) Do you have java/javascript enabled, and is Flash installed/enabled?
3) It could be a bug in Firefox; some kind of memory issue perhaps. Unfortunately it's difficult to diagnose, for me at least, without playing around with it more.

Finally - I have been able to login to linuxquestions after having problems with my userid & password!

Basically, I've resolved this following some of your advice namely installing the Sun Java 6 Plugin. Sun Java 6 Runtime and Adobe Flash plugin were already installed.

Will install User Agent Switcher Extension for Firefox later if necessary.