Do you have extensions that can block Flash content?

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

I have the same problem. On Wednesday, 3/28/2012, the Flash add-in 11.2.202.208 for Firefox 11.0 was installed. Since then, whenever I access my Yahoo web mail (http://us.mg203.mail.yahoo.com), I get the following message: “Your browser is blocking Flash. You must have Flash 9.0 or higher installed and unblocked to run an openmail application. You can download and install the latest Flash Player here.” If I click OK, everything works normally. But the message is a nuisance. My OS is Window 7. I've reinstalled Flash but that didn't work.

Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"

"Remove Cookies" from sites causing problems:

• Tools > Options > Privacy > Cookies: "Show Cookies"

Clearing the cache and removing cookies did not solve the problem. Seems like the problem is rooted in the Flash update because the error messages started immediately after the install of the 11.2.202.208. And the error message makes no sense since I have Flash 9.0 or higher installed.

Here's what happened: Last week, Adobe updated the way they report the Flash version (not Firefox version but the version of Flash) as part of a Flash update. It seems that Yahoo Mail didn't update their code to check correctly and so it looks like (to them) that Flash isn't working (they can see that you have it installed but the version check code doesn't work).

As you probably figured out, you're not actually blocking Flash and this needs to be fixed on the Yahoo side.

Thank you for the great explanation!