You can spoof the user agent (Tools menu) by changing the Gecko engine version and date, but, right now, is most recommendable to just switch to 1.6 beta version, even with bugs
Most major sites are switching fast to latest HTML and Javascript capabilities :/
The custom updated version by JamesD 1.6b2.4 is quite stable, beyond the endemic bugs coming from 1.6b2. Search at the forum for it