If you don't want edit manually user agent, oscpu and platform in "about:config" then blender can do it for you: https://addons.mozilla.org/it/firefox/addon/blender-1/
EDIT: seems that this extension need to be updated, sorry...
But I don't know, if you allow javascript maybe you will leak your real user agent?
I like to switch between iceweasel and seamonkey, and I'm "proud" of my different and unique user agent :) Speaking of privacy, actually I'm more concerned about referer, third-party sites, content delivery networks and so on.
Re:Stop Firefox leaking data about you
by JoshTriplett on Thursday June 25, 2015 @04:52PM
(#9780838)
> But I don't know, if you allow javascript maybe you will leak your real user agent?
The user-agent provided to JavaScript is the same one sent via HTTP.
There is a bug opened (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=748897), but as far as I know, no simple solution exists yet. You can change the user agent with an extension to keep it identical with the most popular Firefox version, but then you have to manually keep it up-to-date.