Do you run NoScript with Firefox? Most web site anomalies occur due to
JavaScript being mangled (not implemented properly).

Get NoScript and install it on all of your Ubuntu machines and your Windows
machines where you use Firefox. With NoScript you can temporarily allow some
features of a website to be applied on a step by step basis as you allow more
and more one at a time or all at a time - temporarily.

If you run through the detailed configuration of all of your Firefox Add-on
configurations - are there any differences in the details?

If there is only ever one or two folks on your home computers, when you install
Java, i.e. Java JRE is all you need to browse with, not the JDK developers kit.
If there is more than one person with an account on any one of your machines,
the usual place to install Java is in /usr/share/java (See Sun's Java
Installation Instructions).

If there is only ever one person on a machine, then all you have to do is
install Java in the /home/<user account> home directory. For example, I have
Java installed in my home directory with the directory, jre1.6.0_15. From
Firefox there is a symbolic link in the directory, ~/.mozilla/plugins:
lrwxrwxrwx 1 ubuntu ubuntu 61 2009-08-24 14:03 libjavaplugin_oji.so ->
/home/ubuntu/jre1.6.0_15/plugin/i386/ns7/libjavaplugin_oji.so

So, check the details of all of your Firefox Add-ons first - where the web page
works, and where it does not. If there are no differences, then check how/where
Java is installed on your machines.

Always follow the Installation Instructions from Sun after you download Java
(JRE) and do not deviate or forget any of the steps.

You don't necessarily need to use Synaptic Package Manager to install Java JRE.