Where Apps are Stored on My HDD (like "Program Files")?

[Jason Moody]

I am really new to Linux. Ubuntu is the first Linux Distro I've ever
used. My question is pertaining to Firefox. I'm trying to set it up so
that when I subscribe to RSS Feeds it will use my reader. But it asks
where the app is located. (like in Windows "Program Files") I Googled it
and have been unable to get the answer. I believe it's /usr/lib but I'm
just not finding it. I'm just wanting to know where the applications I
install are being put on my hard-drive. Thanks for any replies.