That is an indication that you have an incompatible add-on. Follow the procedure in the troubleshooting extensions and themes article to identify the culprit.

Foxit Toolbar is a likely suspect if you started receiving this message recently. The error stopped happening for me the instant I disabled this add-on.

@ghosking

Your above posted system details show outdated plugin(s) with known security and stability risks.

1. Shockwave Flash 10.0 r42
2. Next Generation Java Plug-in 1.6.0_18 for Mozilla browsers

Update the Java plugin to the latest version.

• http://java.sun.com/javase/downloads/index.jsp (Java Platform: Download JRE)

Update the Flash plugin to the latest version.

• http://www.adobe.com/software/flash/about/

This could also be caused by the Ask Toolbar. Can you disable that and try again?

If you have the Ask Toolbar installed under Firefox 4.0b1+ it will give this error. The toolbar lies in its install manifest to say it is compatible with up to Firefox 5.* (you heard me) thus you'll need to disable or uninstall it. A blocklist request is pending...

It happens to me when I installed a new release of skype (5.5). Yust disable the plugin and the problem will fixed.

I concur that the problem is an add-on that is incompatible with FF5. Disable it and reboot. Problem solved.

Thank u very much for your post, cuz u were right on the money! After chasing the problem for hours last nite, installing, uninstalling etc.; I stumbled across your post here thankfully. Thank u once again!