After making a major update to a website, the staffperson who had requested the change told me the website looked no different than before.  I was puzzled by this, because I had tested the website in 4 or 5 different web browser versions to make sure the website looked right & behaved correctly.  But as soon as I started talking with her, I suspected the problem was simply that her web browser had "cached" the old version, and wasn't checking the web server for a more recent version. 

Yup, that's what caused the problem.  And once we went to the Firefox settings to clear the browser cache, it took a good 3 or 4 minutes before the cache was emptied and we were able to refresh the page to see the new version.

Here's how to clear Firefox 3 or Firefox 4's cache, so this problem won't happen to you:

1. At the top of the Firefox window on the Menu bar, click on Tools.
2. When the Tools menu opens, click on Options.
3. Click on Advanced.
4. Click on the Network tab.
5. In the Offline Storage section, click the Clear Now button.
6. Click the OK button to close the Options window.

Now you can click Firefox's Reload button (or press the Ctrl & R keys) to request your web browser get the freshest copy of the web page from the server.

Other Firefox cache options include setting the cache size to 0 (to prevent page & image caching alltogether), or setting Firefox to automatically clear the cache each time you close your browser.