Today when I browses two different forum threads with Firefox, it shows some pr on the threads. But when I copied the url threads on Chrome and visited the threads, it showed pr zero. I also used a pagerank checker tool to find out this and the tool does show pr on the threads. But Chrome is the main browser belonging to Google, I believe Chrome is showing the latest update of pagerank. So my guest is that for pages that have pagerank showed on Firefox but zero on Chrome, this can mean that the pages' PR have dropped to zero where the Firefox pr bar hasn't updated to the latest yet.
Yes that might be the reason, however I would recommend you to go with the pagerank checking tools like checkpagerank.net. Sometimes your the sites you frequently visiting have cookies saved on your computer and they might show you the old pagerank of that particular site. Firefox on the other hand is not so good at checking the pagerank accurately, however if you use it regularly, then I would suggest you to install a plug-in called SEO-Quake. This plug-in gives you exact details of the site you are viewing in its browser
Don't rely on pagerank tools provided in browsers. It is always check the PR with prchecker.info or checkpagerank.net.
Try to clear your cache and cookies of your browser. Log out from your account and then check it again.
I have tried some tools. The pr shown by the tools are similar to Firefox pagerank bar. Only Google Chrome is showing different PR. I trust more on the pr that shows on Google Chrome because I believe Chrome is more update ahead than Firefox. Here's an example url: http://collabtive.o-dyn.de/forum/viewtopic.php?f=8&t=1312&start=10 Firefox shows PR2 but Chrome is PR 0