Make an application default for opening documents

Hello,

I want to use my Thunderbird as default newsreader for rss feeds.
When I click on the icon of the website, it tells me that another application is the default.
There is an option to browse for another application that I can set as default.
But where do I have to browse to find the Thunderbird program.
I have to same problem with pdf -files. I want to use acrobat reader as default.

Thanks in advance

Lainkes

To answer this question better, you need to supply more info - in particular, the web browser and desktop environment you are using?

Anyway, see if these help you:

Default mail client - MozillaZine Knowledge Base

KDE4 file associations « A rather coolish blog