.net is just some system Microsoft created beacuse they couldn't control Java. It's just a framework that makes creating programs on Windows easier, basically. Any program that uses it has to be run with the .net framework installed though. It so happens that some of the feed readers that are separate pieces of software were created with .net, so you may need it for certain software to run. But itself isn't really much to do with rss feeds.
You have three options really if you want to start using feeds:
- get a separate program to grab the feeds for you, and run that when you want to check your feeds. like using outlook for email.
- use the extensions, such as the one you say, in firefox. means you have to run one program less to check your feeds, since it runs as part of firefox
- use a free online tool like Bloglines or Google's Reader. Personally I like Bloglines best; Reader seems pretty good, but it does some annoying things imho. You can just open another tab in firefox to check your feeds with, and of course your feeds are available anywhere in the world; much akin to using hotmail for email
Examples of separate software:
http://www.feedreader.com/
http://www.newsgator.com/NGOLProduct.aspx?Prod ID=FeedDemon
These are of course windows software. Perhaps the best for Mac OSX is NetNewsWire.
Again, I like the bloglines approach, as it allows me to check my feeds from wherever; useful if I'm at uni at the time.
As regards the welcome screen and all that stuff, I'm really not sure what to suggest sorry.