I'm relatively new to Ubuntu & Firefox, and also having the same problem with the Hotmail upgrade (can't reply or compose text in a new mail).
I see the about:config fix suggested in multiple searches, but my about:config settings already appear to be correct, they state Firefox/3.03 in both the 'general.useragent.vendor' and 'general.useragent.extra' values.
Any other suggestions about what to do with this? I've got Ubuntu 8.04.
Also, on another thread someone suggested an addon called 'UserAgentSwitcher' and creating a default profile, but I didn't see any values to fill in when configuring the options on this add on. A plain default profile with blank boxes doesn't do anything for me.
Thanks in advance,