I’m an avid user of Google Reader and Firefox
Subscribe to RSS Feed
Therefore if I encounter an interesting site or blog I usually subscribe to it. In that case Firefox shows the following window by default
A Feed in Firefox
I just discovered that you can click on the option menu and that there you can select Google as your default RSS reader 
. Optionally you can also check the checkbox that will say “Always use Google to subscribe to feeds”. This saves you a click. When you have that checkbox checked and want to change something you have to go to the Firefox Preferences/Options to change it.
Change RSS feed reader preference