You have the Adblock extension installed. Look at the word “Adblock” in the bottom right of your Firefox window, and there will be a solid or dashed underline under it. Pressing Ctrl+Shift+K will cycle it through dashed -> solid -> none.
When either the solid or dashed underline is there, shift-double-clicking will cause the element you clicked on to disappear. I guess this is for annoying advertisements you don’t want to block completely.
When the solid underline is there, shift-clicking will be intercepted by Adblock too, so you can do the shift-double-click without activating a button or link. Unfortunately this breaks things like Gmail’s ability to select a range of messages.
When the underline is gone, shift-clicking does nothing unusual.