Instead of the addons Caleb suggested, you could fiddle with Firefox's configuration. Navigate to about:config
. Use the search bar at the top of the page.
layout.css.devPixelsPerPx
This controls pixel scaling. Change to a higher value to enlarge everything in the Firefox window.
browser.zoom.siteSpecific
This turns off site specific zoom, so one zoom level is used across all sites. Caleb's suggested addons allow finer control, with a default global zoom in addition to site specific zoom.