FastDial's maximum version number is set to 4.0, which you can see on the addon's page: https://addons.mozilla.org/en-US/firefox/addon/fast-dial-5721/

It's possible to change the number yourself. Instructions can be found in the first answer here: https://support.mozilla.com/en-US/questions/837142

If that doesn't work (or you don't want to do it), you can wait for the FastDial developers to update their addon.

If you don't want to wait, there are alternatives like New Tab King and New Tab JumpStart that seem to do the same thing, more or less.

Basically, I got Fast Dial back by downloading the beta version. Even this suggests that there's nothing wrong with Fast Dial, just that things have been taken out of the hands of developers especially in FF5 so they are frustrated somewhat. Regardless, you can get Fast Dial back with FF5 by going to this url: https://addons.mozilla.org/en-US/fire.../versions/ Enjoy.