By default, Firefox should remember each zoom setting per-site between sessions. If you've set the Firefox privacy options (Firefox button / Edit menu -> Options -> Privacy) to not remember history or to clear it on exit, you may have it set to forget your zoom setting. If you're using custom privacy settings and have "Clear history when Firefox closes" turned on, the option specific to clearing zoom settings on exit is the "Site Preferences" checkbox in the "Settings" button. Likewise, if you manually clear your history and clear the site preferences via the Clear Recent History dialog (check in its details), that too will clear zoom settings.

If you don't have Firefox set to forget all history or site preferences, and it still forgets, you may have a broken addon messing things up, Troubleshooting info is here: http://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

If you would prefer all sites use different font settings, you can go to the "Content" tab of your Firefox options, click the "Advanced" button in the "Fonts & Colors" section, and use the dialog that comes up to set a minimum font size to be used for all sites.

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/
• NoSquint: https://addons.mozilla.org/firefox/addon/nosquint/