Firefox browser is in the 3.X versions with new fixes and updates being quite regular now. Now with the latest version (3.5.1) vulnerabilities were fixed here is one working addon which allows you to change Firefox 3 interface into the older version of firefox.
Wine Stripe Realfox is the firefox skin addon that transform the look and feel of firefox browser. By installing this addon, your firefox will resemble the older Firefox 1.5 version skin. It removes all the unnecessary cluttering buttons, extra icons from the address bar.
Before installing Wine Stripe addon on Firefox
After installing Wine Stripe addon on Firefox