Myk Melez has been working on some innovative ideas related to persistent site-specific preferences and their control. His current prototype is a Firefox extension which can be downloaded through the Firefox Add-ons site (here), or directly from his website, here. Currently the extension only includes a text size control, but Myk will be adding more features over time.
For more information about Myk’s experimentation with this concept, see his weblog post. If you have thoughts or feedback, Myk has suggested using the dev.apps.firefox list/group, or just email him directly at myk-at-mozilla-dot-org.