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.
Source
Fix for installing unsigned add-ons in Firefox Dev and Nightly
