An answer that has worked for me ...

A big THANK-YOU to dougcoull with his answer to the question “How come Vimeo won’t work with Firefox 4”. (Search Vimeo in FF Help).

Type "about:config" in the address bar of your browser to take you to the config settings.

Scroll down to "dom.ipc.plugins.enabled.npswf32.dll" or cut n paste the term in the Filter.

If the entry is ‘false’, double click the entry to change from 'false' to 'true.

Close FF and then re-open for the setting to take effect.

Please take the effort to inform dougcoull if it works for you too.

Only issue for me is you may then find that plugin-container.exe is now running with Vimeo eating up and hogging CPU :(

had the same problem. in Firefox 8, the property is "dom.ipc.plugins.enabled", and must be set to true.