This should be fixed in Firefox 25 as part of bug 768651. I think the easiest workaround until 25 ships (aside from running Firefox 25's beta/aurora builds) would be to disable plugins in the instance of Firefox after you start it up with cfx run.
Status: UNCONFIRMED → RESOLVED
Last Resolved: 5 years ago
Resolution: --- → DUPLICATE
Duplicate of bug: 768651
It's still hanging for me on youtube.com with Firefox Setup Stub 25.0b7
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