You may have accidentally zoomed webpage(s). Reset the page zoom on pages that cause problems.

• View -> Zoom -> Reset (Ctrl+0/Command+0 (zero))
• https://support.mozilla.org/en-US/kb/font-size-and-zoom-increase-size-of-web-pages

Note that on Mac you can zoom the page if you hold down the Command key while rotating the mouse wheel.

• about:config => mousewheel.with_meta.action

Meaning of the Action:

0: Nothing happens
1: Scrolling contents
2: Go back or go forward, in your history
3: Zoom in or out (reflowing zoom)
4: Treat vertical wheel as horizontal scroll
5: Zoom in or out (pinch zoom)

Thanks for the tip. Zooming does work to make the page larger in the window, but the proportions are off. The page is more narrow, like a 640x400 monitor, rather than wide screen. The image area on the page is narrow, the margins on the sides are wider than before and on other tabs. The image itself can zoom, text and graphics become larger or smaller, but the width stays the same (too narrow). Does the code from the website define the width of all pages on that site? Maybe these pages were always narrower but I didn't notice until this zoom issue came up?

I retried the cmd 0 and that does seem to make it more like before. I think I will try this and see if it is the answer.