OK, after some off-forum discussion, the site is a table with a width set to 1250px. Where I am the display is uniform across all browser/version and despite changing monitor resolution - no obvious code issue. An interesting mystery!
Laura, some terminology - very general as you've said this is new - anyone else please feel free to provide a better explanation.
1. Page/Document width - that's the width you've specified in your code, which for this document is 1250px. Whether that's a good width depends on your target "user", and that is about their expected viewport size and monitor resolution.
2. Viewport width - the browser width, less the "chrome" (scrollbars etc). The width varies depending on monitor resolution, borwser scrollbars have different widths, etc. (But scrollbar widths won;t account for thr 25% difference here). The other challenge is the user: Grab your browser and reduce it's width by 1/2 the screen - you just 1/2'ved the viewport width - it's that simple ;).
3. Monitor resolution. That's the setting applied to monitor. If you're on windows, find it under Control Panel / Display / Settings. If you haven't thought about this yet, it makes a big difference. For example my eyes get tired so I'm on 1024px wide - a site with width=1250px has a scrollbar. If I change the setting to 1280px the scrollbar isn't required - as you would expect.
... no, it's not that simple :) - for example, all the issues Lucy referred to - but hopefully enough to start.
To summarise, this is a 1250px fixed-width page that is forcing a horizontal scrollbar in firefox on every screen except one.
To eliminate the first variable - is your montior resolution wider than the others? Second, can you just confirm this is definitely not happening in ie or chrome - but is doing it for a maximised firefox? And when you return, can you also provide the versions?