While it's a major improvement on the more sluggy Firefox builds, it does however add a slight inconvenience for developers by restricting addon installations to signed ones. In my case, it was Firebug 3 alpha.
If you find yourself needing to install an unsigned addon, you can quickly do so by:
- Type "about:config" into your address bar
- Confirm OK "I'll be safe"
- Filter to find "xpinstall.signatures.required"
- Double click it to set the value to "false"
You shouldn't need to restart for this to take effect. Try installing the addon again, it should work this time.