Many internal company applications require some component, or extra installation to be done before the application can be used by employees. There are many Web sites that use content that need a plugin. For example, there's lots of Flash, Quicktime, Real video files, Adobe PDF's around. To view them, you either need to install a plugin or perhaps it was included with the browser. Often, a certain version is required (Flash 6, for instance). For internal corporate tools. there's all kinds of speciaalized components that are used. Many applications are written in Java, or Python or whatnot, and require a runtime of some sort to be installed.
Nobody seems to have a problem with requiring that users have such plugins installed. Web sites will just point you to where they can be downloaded. When it comes to browsers, however, people seem to have a notion that browser usage is a either-or thing. You can use either one browser or another, but you can't use both.
But who says you can't use lots of different browsers? Do I even need to have a default browser set? If an application that I need to use wants an Opera HTML/CSS engine, I'll download one and use it. Why does it matter that I don't use Opera to browse the web? There are all these web-based applications that seem to demand that you be using IE as well as some specialized ActiveX controls. But is it really a web application, if it has such requirements? What's the difference between that and requiring that someone be using Mozilla Firebird?
In the last poll, someone seemed to think that XUL couldn't be used by people who normally use IE. This isn't logical. There's no reason an application couldn't insist on the appropriate browser or browser component to be installed, as with plugins. Many XUL features will work, for instance, in the Mozilla ActiveX Control. There are many applications that require downloads first. Why should browsers be any different? On one hand, I could insist that someone have IE 6 on Windows, with an extra component installed to do more specialized UI, or I could just require they have Mozilla and write the application in XUL.
Is it a download size thing? Surely not. Both Opera and Mozilla Firebird are about the same size or a smaller download than Real Player, Quick Time, Acrobat, and many other plugins.
This is why I think a proper GRE is a good idea. If Gecko could be treated as a simple component that can be used with other architectures, it will better be able to distance itself from being a browser that people think you can only have one of.