Firefox Theme Installation:

On all non-official Mozilla sites Firefox requires you to specifically allow the installation of add-ons.


Firefox 2:

When you click on the "INSTALL" link a yellow bar pops up:




Click on "Edit Options...", then on the "Allow" button:
Now you should be able to install the themes.

Firefox 3:

In Firefox 3 you can click directly on the "Allow" button:




If you don't want to click the "Allow" button all the time you can also add www.theme-oasis.org to the list of websites that are allowed to install add-ons. For that go to "Options" --> "Security" --> "Exceptions..."




Thunderbird Theme Installation:

For installing Thunderbird themes you have to download the .jar files somewhere on your computer.
(Right-click on the "DOWNLOAD" link --> "Save as...".

Then open the Addons-Manager, choose "Install", and locate the downloaded file:








Windows XP Theme Installation:

Windows XP by standard prevents the installation of non-Microsoft themes. To fix this there are two ways:

Method #1:

This works by patching the uxtheme.dll of Windows XP with a modified version, to allow the installation of third party themes. You can use for example the "Uxtheme Multi-patcher" to do this:
http://www.softpedia.com/get/System/OS-Enhancements/UXTheme-MultiPatcher.shtml

Update: The Multi-Patcher 5.5 so far has no support of Windows XP Service Pack 3.
You can try following patchers instead:
http://www.syssel.net/hoefs/windows/uxtheme.php?lang=en
http://winprj.net/board/viewtopic.php?f=7&t=216&sid=a1bc5cca053af7a2483c0f4784ab888c

If they work depends on your Windows build and language version.

Installation of "OldFactory XP" after the Patch is done: Extract the "OFXP.zip" into C:\WINDOWS\Resources\Themes Make sure to retain the folder structure of the zip file. Now right-click on your desktop and select "Properties". In the "Themes" tab you should see now the "OFXP" theme. Just apply it.

Method #2:

If method #1 doesn't work for you, or if you don't wanna patch your uxtheme.dll, then you can also use the software "Style XP" by TGT Soft.

After the program installation you can simply install the theme by importing the downloaded .zip file.

A trial version can be downloaded here:
http://www.tgtsoft.com/download.php

The downside is that you have to pay for the program after the trial period is over.