After updating to Firefox 31, I had lots of errors - about 2 a second - as Firefox refused to let the command at line 4 of inject.js run.
This makes things very sluggish, and makes debugging a web page using Firebug impossible. :(
This is under windows 7, 64 bit - with all updates. Have checked for virus's and malware and the machine is clean.
As a temp fix I have disabled zone alarm's add-on in Firefox.