If you are a Firefox user I suggest that you head on over to Mozilla.org and grab the latest security and stability update. The patch fixes 10 security holes, three of which are rated critical.This update can also be applied manually by selecting "Check for Updates" from the Help menu.

MFSA 2008-11 Web forgery overwrite with div overlay
MFSA 2008-10 URL token stealing via stylesheet redirect
MFSA 2008-09 Mishandling of locally-saved plain text files
MFSA 2008-08 File action dialog tampering
MFSA 2008-06 Web browsing history and forward navigation stealing
MFSA 2008-05 Directory traversal via chrome: URI
MFSA 2008-04 Stored password corruption
MFSA 2008-03 Privilege escalation, XSS, Remote Code Execution
MFSA 2008-02 Multiple file input focus stealing vulnerabilities
MFSA 2008-01 Crashes with evidence of memory corruption (rv:1.8.1.12)

Update: A few hours after the release, a hacker has discovered the flaw, where he recommends using the NoScript plugin. In the mean time you can either use another browser, or install the NoScript plugin to mitigate these issues.