This article explains how to clear the Firefox cache in order to troubleshoot Yahoo! Messenger for the Web

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In order for Messenger for the Web to function properly on your system, use the following steps to make sure your system is up to date and properly configured.
First, download the latest updates for Firefox, Java, and Adobe Flash Player. These can be accessed at the following links:

• Mozilla Firefox
• Java Runtime Environment
• Adobe Flash Player

Next, please clear your browser's cache:

1. Click the Tools menu and select Clear Recent History...
2. Click the drop down menu and select Everything.
3. Click the arrow next to Details.
4. Place checkmarks on all six options.
5. Click the Clear Now button.

http://help.yahoo.com/kb/index?y=PROD_MSNG&page=content&id=SLN19