As I tried to explain (and obviously didn't succeed) in my last message, Joomla websites are constructed in PHP which is used as a
server-side scripting language.
Firebug, on the other hand, merely exposes the objects detected by the client (i.e.
Firefox). If this sounds confusing, don't worry.
Think about it in these terms: when you buy milk at the shop what do you get? You get milk. But, even though you can analyse the milk to determine the percentage of fat, water, lactose and whatever else goes into making the milk, you can't find out the details of which cow in the world was responsible for producing the raw ingredients, can you?
The same thing applies with server side scripting. At the client/browser end, you can't tell what, specifically, was responsible for producing the raw HTML that was transmitted to you via the internet. The only way you can do that is to examine what is going on at the server.
Firebug can't do that.
As for finding the name of the file that contains a certain piece of text, if you don't know which file it's in then, I'm sorry, you'll have to search all of them (or at least, start by knowing where to search). If the text is generated by
Kunena, there's a good chance that it will be in the
../components/com_kunena folder (or one of its subfolders)
somewhere (unless someone else knows where what-you're-looking-for is). What were you looking for again?