blocking web browsers

One way (and perhaps the only way) to identify a browser client is to extract the http request headers from browser traffic. As far as I am aware, the Fortigate does not have that ability. (Besides -- such header info can be faked.) What you' re requesting is better suited by using a client-side based solution (e.g. FortiClient) or other application/software based policy (including locking down workstations to prevent unwanted software from being installed) that restricts or prevents those client browsers from accessing the Internet. This is not perfect but you can always set up a user authentication method in which where you can simply edit (code) the Login or Login Challenge page to ensure the user is using a supportive browser.

Actually there are signatures to block each browser . Here a complete list Though I think except for " HTTP.BROWSER " you need to enable " extended package" before you can use them http://www.fortiguard.com/applicationcontrol/app_pop_risk.html#name=http.browser HTTP.BROWSER.Firefox HTTP.BROWSER.Nokia HTTP.BROWSER.Dolphin HTTP.BROWSER.Flock HTTP.BROWSER.Android HTTP.BROWSER.Opera.Mobile HTTP.BROWSER.SeaMonkey HTTP.BROWSER.BlackBerry HTTP.BROWSER.IE.Mobile HTTP.BROWSER.Opera HTTP.BROWSER.Safari HTTP.BROWSER.Chrome HTTP.BROWSER.IE HTTP.BROWSER