yahoo, yahoo news and such show the center third, but mail stretches full screen.

not sure if that helps, just trying to give as much info as I can.

Can you give a screenshot?

here are some screen shots

What are your screen dimensions? Is that a 1920 display?

Note that a lot of web pages do not automatically adjust their content to fit on the screen by using percentages for all elements and in such a case Firefox won't fill the screen, but leave white space at the left or right and center or left align the content. You would have to zoom the pages to make it use a wider screen estate and fill the entire width.

You can use an extension to set a default font size and page zoom on web pages.

• Default FullZoom Level: https://addons.mozilla.org/firefox/addon/default-fullzoom-level/
• NoSquint: https://addons.mozilla.org/firefox/addon/nosquint/

Zoom does make the width a bit but it makes the text larger. the text starts to wrap.

I tried making the text larger, but that did not increase line spaces so the text started to overlap.

chrome does a lot better. the zoom does fill page more. oddly, using chrome on here, the menus for this store fill the page. the content below does not.

I am trying to find what the resolution is on my monitor. it is a 21.5" wide. Dell PC with a emachines monitor.

You need to use full page zoom (i.e. remove the tick: View > Zoom > [X] Zoom Text Only) to make Firefox zoom the page and not the text. Otherwise only the text is zoomed and other elements stay the same.