Make sure you have one combined feed as well. Some potential subscribers may want it all.
1.When i complete my code for rss (and upload it on my server), i have to create rss files for every category with some default articles in it? When a user publish an article it's better to update the current rss for this article's category or replace it with a new rss file with this only one article in it?
I think you've figured this out, but you'd want more than one article in a feed. I have some feeds with hundreds of items in them. It's up to you and the frequency of your updates.
An other question i have is if a user subscribe to an rss, lets say just with firefox, how and when firefox will inform the user for a new update?
That varies. Feed readers will normally have a set update interval, but that can often be customized by the user. You could expect anything from almost real-time updates to hours or days between pings.