From MozillaZine Knowledge Base

A bug can cause Firefox to report that updates are available even when none exists. One of these methods should fix it.

Method 1 Close Firefox and delete active-update.xml, updates.xml and the updates folder from one or both of these locations:

• The Firefox installation directory, for example, C:\Program Files\Mozilla Firefox [1].
• (Firefox 2.0.0.2 and later) The folder where Firefox stores temporary application data for your operating system: [2] [3] Windows 2000/XP: C:\Documents and Settings\<username>\Local Settings\Application Data\Mozilla\Firefox\Mozilla Firefox
  Windows Vista: C:\Users\<username>\AppData\Local\Mozilla\Firefox\Mozilla Firefox. Important: These locations are hidden by default.. Read this if you need help viewing or searching Windows hidden files and folders.

Method 2: Download the most recent version of the Firefox setup file from www.mozilla.com and install it "over the top" of your current Firefox installation. [4] [5]

See also

Software Update not working properly

Related bug reports

• Bug 368661 – on install, remove the updates directory, updates.xml and the active-update.xml file
• Bug 372981 – Update history missing after update to 2.0.0.2
• Bug 374140 – Software Update (Firefox 2001 -> 2002) Problem - wrong User Agents
• Bug 375710 – Firefox wants to downgrade to 2.0.0.2 from 2.0.0.3
• Bug 376305 – scenario where the user could get prompted to update to 2.0.0.3 (even though they have 2.0.0.3)