You can maximize the Firefox window using the usual rectangle button to the left of the red X button.

By itself, this won't expand the page content. For that, you can try using Firefox's zoom feature. (More info: How do I use Zoom? | How to | Firefox Help).

If you want to change the page's margins without enlarging the text and images, that option isn't built in to Firefox. You might find an add-on that lets you tweak the site's design so it expands horizontally only. I haven't researched whether any of those exist or how well they might work.

Any luck?

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/