This has been fixed in Firefox 10 (or 9?). Until then you could use this extension:
https://addons.mozilla.org/en-US/firefox/addon/add-on-compat...
If you are responding to "only can be used in FF8" the font information extension only works in FF8+ because of the api hooks needed are not available in previous version. It's not a min/max versioning issues.
Here are some extensions I found critical to my sanity to keeping things somewhat familiar:
and Then once you are in FF8 treat yourself to extensions you can only running in FF8+ like Font Information which shows you what fonts are being used on the page in the page info dialog.Now my FF8 looks a lot like 3.6 but works several times faster.