A weakness and multiple vulnerabilities have been reported in [Mozilla Firefox], Thunderbird and SeaMonkey, which can be exploited by malicious people to disclose potentially sensitive information, conduct spoofing attacks, bypass certain security restrictions, and compromise a user’s system.

The weakness and the vulnerabilities are reported in [Firefox versions prior to 19,] Thunderbird versions prior to 17.0.3 and SeaMonkey versions prior to 2.16.

These are marked as “Highly critical”, and it’s therefore suggested that you update your software as soon as possible.
You can find the updates here:
SeaMonkey: http://www.seamonkey-project.org/
Thunderbird: http://www.mozilla.org/en-US/thunderbird/
Firefox 19: Either simply go to Help, About in your browser to download the latest version if it didn’t already auto update, or visit http://www.mozilla.org/en-US/firefox/fx/#desktop

Firefox 19 also contains their all new built-in PDF reader, which means that both Chrome and Firefox now has built in PDF readers. Those wanting to secure themselves from last week’s Adobe Acrobat Reader vulnerability could therefore choose Firefox as the alternative as well.

More information:
http://secunia.com/advisories/52280/
http://secunia.com/advisories/52249/
http://secunia.com/advisories/52286/