Disappears randomly in latest versions, but is restored after quitting FF window (not the app) and then reopening.
More important, graph doesn't fit inside page, can't be scrolled and randomly becomes much too small, or much too large and is unusable.
Also why no more list view?
Almost completely useless, ready to uninstall.
OS X 10.11.6/FF52.4 ESR
EDIT: Discovered that graph can in fact be zoomed in and out with mouse dragging left or right or up or down. Takes effect after some delay, but deals with the clipping issue and makes Lightbeam usable after all. Revising my rating. Many thanks to the developer, who has been extremely prompt and responsive.