Firefox 3.0 remembers what zoom level you last used for each domain. If you just want to reset the current zoom level for the page in question, hold down Ctrl and + (zoom in) or - (zoom out) on Windows, Command + / - on Mac (I think?) until you reach the desired level of zoom. Ctrl/ Command and the 0 (zero) will reset the zoom level to the default. Close your browser after setting the zoom level and the setting should save.
If you don't want Firefox to remember zoom settings per domain,
In the Location Bar, type
about:config, click on the "I'll be careful, I promise!" button only if you are comfortable editing advanced settings.
If you proceed, search / filter or scroll down to the setting labeled "browser.zoom.siteSpecific" and double click on that setting to set the value to "false".