With the Google Pagerank extension! This is a Firefox 0.9.x compatible extension that adds a Google PageRank widget to the statusbar.

it acts exactly as the real googlebar on IE: calculate a checksum, send a request to google.com (with the googlebar User-Agent string), and the reply contains the pagerank.

Screenshot of Google PageRank extension in action

Grab it from installation section or the author's website. (Note: Firefox should prompt and say something to the effect that the installation has been blocked. You can trust this extension, so click on the "Edit Settings" button that appears and allow the installation.)

It updates when you switch tabs (it could take a while depending on your Internet connection), in case you were going to ask.

Screenshot of Google PageRank extension in the extension manager