I notice that even though I have uninstalled addon from Firefox, the settings remain in about:config. No way to remove them right? seems un-necessary to have the lines around.

Just-right click on them and select Reset.

Gosh so many to one-by-one to reset. why don't uninstalling the addon also remove their configuration lines?

Because the extension authors didn't take the time to code to remove it. I also haven't done that with my extensions either...it doesn't hurt anything though.

Alright then