PageRank is not dead, and you can still get plugins for Firefox, which will show the PageRank of the current URL. One of such plugins is called searchstatus, and it also supports Firefox 9.
PageRank and Firefox 9
Google changed the URL used to fetch the PageRank, which is why your plugin may have stopped working. Searchstatus is one of the only plugins which have updated the URL. You can however do it manually for many other plugins, given that you know what files to edit.
PageRank on other Browsers
The google toolbar still works in IE9, but the PageRank button has been moved into the "more" dropdown. To get it back on the toolbar, simply right-click the button, and click "Add to toolbar".