The release of the new add-on manager in Thunderbird 5.0 has demonstrated that many people are not comfortable installing an add-on in Thunderbird, a part of this is the fact that the process of installing from a file is far from intuitive, there are no associations for the file and double clicking it get the standard dumb answer from the operating system asking what to do with this odd unknown file type.
The second problem people encounter is that Firefox uses exactly the same XPI file format, so if you browsing in Firefox, simply clicking on an XPI link on the web prompts the file to be installed in Firefox. It is for this reason that almost all download sites display a message similar to this one.
Add-ons in Thunderbird can not only be simple, they are the core of what makes these Mozilla Applications fit for the needs of almost anyone. There are hundreds of add-ons for almost every conceivable niche function but they are greatly underused simply because people either don't know they exist, or are put off by the whole process of installing them.
Add-ons can change the theme (or skin) to introduce more colourful icons etc. or they can add functionality. To get the full scope, take your browser here and explore the categories shown on the left of the page. These are only the add-ons hosted on the Mozilla add-on site. You will also find many other add-ons, which are not on the site but references in support forums or Blogs such as this. One personal web site that contains a whole host of useful add-ons is KAOSMOS WEBSITE
There are two ways to install an add-on.and I will talk about the simplest first.
Go to the add-on manager. You will find this on the tools menu. It opens a new tab, which in my case flickers several times when it is opened. From here you are presented with a screen offering some of the more popular themes and add-ons. There is also a search box, and a hidden menu, and it is this that is the most useful part of the process. In the search box type in the name of the add-on you want to install and it will locate it (sometimes) and list it with an install button. Nothing could be simpler.
The second and far more common approach is to install an add-on from a downloaded file. There are two main reasons for this. People link to add-ons on the web, and most of us use a browser to find out about things. Your browser will download the file, but as I said earlier there will be no help from the operating system in installing the file into Thunderbird. It is assumed that if you downloaded the file you know what to do with it.
So what to do. Again it is simple, but only once you know how to do it.
In the addon manager there is that hidden menu I refered to before and it is using this menu that you install your XPI file to Thunderbird