only "picked" area of page visible

I went to my favorite web site this morning (dpreview/forums) and when it displayed I must have accidentally clicked some Firefox thing because the page was then overlaid with a thin orange highlight with the word PICK on it. I tried moving the cursor off the page but it only changed which segments of the page were highlighted. So I clicked and the page was replaced by just the menu section of the page, apparently that was what was highlighted when I clicked. Now, no matter what I do, when I try to go to that url only the menu is displayed, the normal list of forums is missing. I've restarted FF. I've rebooted. I've cleared cookies and cache. I've refreshed the page. Nothing helps. Other web sites are fine. I can't imagine what this is for. A diagnostic tool or accessibility thing perhaps. Please help. I'm running latest FF on latest Win 11.