Where to find add-ones user agent overrider for firefox?

[496]

follow Mr.eye's instructions & one more thing i forgot to say-
right now your settings is
Tools>Default User Agent>internet explorer>internet explorer 7

sites like youtube won't work with this settings so while using other sites set your settings to
Tools>internet explorer 7>Default user Agent

and refresh the page,done

you don't have to restart the pc for any of these,after installation is complete it will show the message 'restart firefox',click it & only the firefox will be closed & restarted

if you are unable to do these,must be some problem with the firefox,uninstall it,download & install the new version of firefox, ,

i'm using the version-'firefox 23.0'
it is not new but there was some features missing in new firefox so i'm still using little bit old one,