Step 1 : Backup Everything , Although the step doesn’t involve anything risky AFAIK, I still tell you to backup and release myself from liability. In this case just remember the key value that you are going to edit or even better make it blank!
Step 2 : Type about:config in your address bar
Step 3 : Right Click on any entry, Select new string and enter string name as ‘general.useragent.override’.
Step 4 : Take Your Pick Mozilla Firefox versions from http://en.wikipedia.org/wiki/User_agent#Browsers
You might want to replace the version at the end to suit your version of Firefox.
Click on the image below to see the error Yahoo Mail Beta gives, i uninstalled Ubuntu so i cant confirm whether its started to work there, but people say it works in Dapper, See comments also. I can confirm though it doesnt work in Windows Me.