They're still working on fixing that Bug - https://bugzilla.mozilla.org/show_bug.cgi?id=608589 - a fix for another Bug - https://bugzilla.mozilla.org/show_bug.cgi?id=613888 - landed earlier this week which might have fixed it. Unless you are willing to install a nightly Trunk which has the latest fixes, you'll have to wait for 4.0b10 for the fix.

Thanks for the prompt reply, glad to hear they are aware of it and working on it.

The only add-on that generates this error message for me is Tab Utilities 1.0. I have tried all kinds of variants on the add-on settings with no success. Would really appreciate having this fixed (as I am sure ithinc who wrote Tab Utilities would too). At first I thought an simple setting was the issue, but I was wrong.

I posted some firefox 4 themes for Stylish that were causing this box because the mouse-over opacity of the inactive tabs was set to a value less than 1.0 I updated my Stylish themes to fix this issue with them. I'm not sure if this is an issue 3rd party coders will have to work around or something that Firefox will fix on their end, but I thought I would post this in case it helps.

I have Tab Utilities and Colorful Tabs addons installed, disabling Colorful Tabs seems to fix the problem. Could this be an interaction between other addons too?

Btw, Tab Utils is now at version 1.0, but auto update still shows 1.0 rc5. Updating to 1.0 does not fix the problem with Colorful Tabs.

Thanks for the tip! I disabled Tab Utilities and the error went away :).

i had this problem when using Tab Utilities, i went to about:config and found browser.tabs.animate and changed it to false and the problem is gone.