This is one of those little things that has bugged me for quite some time, but not quite enough to do anything about... until now... 

Whenever I try to view an xml file, whether it be a sitemap.xml file from one of my website projects, a data export file, or an rss feed, Firefox shows me a screen with two options, "add to google reader" or "add to google homepage" ... usually I want neither, I just want to see my xml!

The fix was actually super simple... but, lest I forget at some point ( most likely not too far in the future) how I managed to banish this little annoyance, I'll post it here:

1) In FF, go to Tools > Options in the top menu
2) Select the Applications tab
3) Scroll down to "Web Feed" (how annoying ... not 'xml', not 'rss', but 'web feed'? really? )
4) Change the Web Feed 'action' to "Preview in Firefox" (mine said "Use Google" before changing)
5) Click the 'ok' button and go to any .xml file url to see all the wonderful XML , sans reader-prompt annoyance !