I've been using Gmail within Firefox for over a year now. Starting two days ago, Gmail no longer auto fits within the size of my Firefox browser tab (see attachment). I now have to scroll down every time I want to attach something to a new email I'm composing, I have to scroll right to be able to read the entirety of any of my messages, scroll left to see my "labels" (categories) on the left, etc.

Do you know why this suddenly started happening? And how I can get it back to its normal behavior, the way it's been for over a year? None of the other websites (Google Contacts, Calendar, Slack, Yahoo, etc.) are behaving this way - only Gmail.