Yes, it is. But you may see a button telling you about a forthcoming upgrade @ http://ca.features.mail.yahoo.com/ At the foot of that page, you'll find a link to Firefox 4.0, so I assume they've tested the BETA version with Firefox already.

Thanks for your help Xircal

You're welcome.

When i go to the new Y!Mail, this is what it shows (pic), ALWAYS. so I have to refresh the page. Facebook isn't able to work properly with Firefox 4 as well. You can't post a comment (which you have to press Enter now that they've changed it recently so I'm guessing certain shortcuts on certain websites don't work in Firefox 4), upload photos, and delete previous posts. Hope this problem gets resolved.

@ xaniz25, try running Firefox in Diagnose Firefox issues using Troubleshoot Mode. If it functions properly in that configuration, then one of your add-ons is the culprit.