Firefox update just broke something

ExtJS 4.2.0

I haven't changed my code. It was working in Firefox before leaving the office. I come home and start up Firefox and it does and update. Now my application hangs (sometimes) and I get a message in Firebug stating ....

Relative positioning of table rows and row groups is now supported. This site may need to be updated because it may depend on this feature having no effect.

Anyone else?