i don't know where you got the impression. if anything it's the other way round, no?
Of course this requires that you have it installed beforehand, otherwise just download the files manually and edit install.rdf before installing it.
Which seems to mean that you can't run the stable version of pentadactyl on Firefox 27 (the current release) [1]. I do understand that it is not equal to being inactive, but at least for me it was one of the reasons why I returned back to vimperator (which has a stable version that works for Firefox 27).
[1]: https://addons.mozilla.org/en-US/firefox/addon/pentadactyl/
