Firefox is really a great browser with tabs , plugins, Anti fishing, RSS, … and many more things.
This system allow people to register web service to open mime type. So you can open a file without local rich client to open it. The power of web service will do it for you.
Imagine you want to open a document .doc, but you do not want to install MS word, you will use the Google document web service to open it!!
This system is great because no issue about multi-OS support. Most of plate-form have a browser and this is the only thing needed!
PDF file is a pain to open because Acrobat reader is really slow and have to many module to load whereas it is quite never used. I have never understand why acrobat open all module even if not needed and why it will not check file content to see what is needed… I use Foxit reader to open PDF but now I will use WebService to open it.
Your computer will no more need extra application than a browser and access to web;)
A lot of web service is outside to help you to open document. In my opinion, it is the future for a big part of application. But this system is not perfect to do complex things (coding, compile, .. for example) but it is cool to view any format. So, you will no more need viewer because all will be online.