Today my girlfriend came to the stunning conclusion she was unable to reply on e-mail messages when she was using here Ubuntu box. After some investigations she found out that she was already running the latest version of FireFox and she was still prompted, after the login page of hotmail, that she was not running a supported version. it finally came out that the version installed was a 'ubuntu release' of firefox.
Following the steps below will make the 'not able to reply' problem disappear.
1. Type about:config at firefox URL field and press enter.
2. Click on "I'll be careful, I promise!" button.
3. Then at the top filter field, type useragent to make "general.useragent.vendor" name in sight.
4. Double click "general.useragent.vendor" to change the value from Ubuntu to Firefox.
Thanks to John Navarro on ubuntuforums.org