It's working for me using Firefox 32.0 + Firebug 2.0.4 on Win7.
According to
another post from 2011 a reinstallation of Firebug may fix the problem in this case.
If that doesn't help, please check whether it's an extension that conflicts with Firebug. To do so disable all other extensions and verify whether the Enter key is working again. If so, enable the extensions again one by one and test after each one whether it's working or not to find the culprit.
If deactivating all other extensions doesn't fix the issue, you may follow the other steps at our
first aid page to solve the problem.
Please let us know what you found out.
Sebastian