You propably got already used to Firefox’ zoom feature. Firefox 3 introduced a new setting that enables it to remember the zoom value when you visit a website. Whenever you re-visit the domain, the previously used zoom level will be re-applied.
If you don’t like this feature, open the about:config page and toggle the boolean setting browser.zoom.siteSpecific to false.
Of course, you can always use Strg-0 to reset the zoom to the default value.