A minor update according to the version number, but an important security update, none the less
Security Fixes in 2.0.0.4:
MFSA 2007-17 XUL Popup Spoofing
MFSA 2007-16 XSS using addEventListener
MFSA 2007-14 Path Abuse in Cookies
MFSA 2007-13 Persistent Autocomplete Denial of Service
MFSA 2007-12 Crashes with evidence of memory corruption (rv:1.8.0.12/1.8.1.4)
Fire up Firefox (it is not your default browser?) and a dialog window should open inviting you to update. Or go to English English for a direct download.