Download the iWeb patch from here to your desktop.
Double click the downloaded zip(Fix_iWeb_and_Firefox_3_1.1.zip) to extract the application.
Double click the extracted application (Fix iWeb and Firefox 3 1.1) to begin.
A popup will appear asking you to confirm the open of this downloaded application. Click the Open button.
You will then be asked to enter an administrator username and password. Do this, then click the OK button.
Once the application has finished patching your iWeb files a popup will appear letting you know it has been successful. Click the OK button.
Now that iWeb has been patched please republish your iWeb site and clear your Firefox cache. Your iWeb Gallery will now display correctly!