Mozilla identified some security vulnerabilities with Firefox 16, released earlier this week. The 16.0.1 update addresses said vulnerabilities. Firefox 15 users are not affected by these vulnerabilities. Another update, Firefox 16.0.2 is still being evaluated as Mozilla is still determining the full scope of the window load handler failure issue which causes some add-ons not to load when a new window (not tab) is opened.
Existing Firefox 16 users can updated to 16.0.1 via Help > About Firefox or download and install Firefox 16.0.1 manually via the getfirefox.com site.An update for Firefox 16.0.1 for Android has already been pushed out to Google Play Store.