Strange problem with Firefox 3 ā€“ return key not working on address bar

Since upgrading to Firefox 3, I’ve had an intermittent problem, both on my work and my home PC’s. Every now and then the enter key will refuse to submit a URL entered in the address bar.

The strange things is that selecting a bookmark from the bookmarks drop down, or from the bookmarks toolbar still works as normal. A restart of the browser does of course fix it, but it seems an odd little bug.

Anyone else experience this?

UPDATE

A number of people in the comments below have reported that disabling some of the plug-in may affect it. Such as the Delicious or AVG Safe Search plug in.