Firefox middle-click new tab in background freezes browser

[snowcousins]

I've got Firefox 28.0 on Ubuntu 12.04, and every time I middle-click a link to open a tab in the background, the browser freezes, the window dims for about 30 seconds, and then the tab loads. Occassionally I get the following script error message with a prompt to either stop the script or continue: Script: chrome://browser/content/tabbrowser.xml:4552

What am I doing wrong?

Thanks!

[cariboo]

Remove the Solved tag from this thread, as it looks like it is still unsolved.