By typing about:config in the address bar you should be able to configure a proxy. You can use the search bar just below the address bar to find only preferences that mention proxy. Also the settings should be the same in the desktop version so setting the preferences in the desktop ui should then opening about:config in the desktop version should help you get the about:config preferences for mobile set right.

sorry there was some mistake in problem statement.actually i want to configure proxy in Firefox which is installed in my android phone . i know very well that how can i configure proxy in my laptop or desktop problem is with browser on my android phone

The instructions above for "about:config" will work in Firefox for Android, as well as Firefox for desktop/laptop.

You can also install this add-on in Firefox for Android, for an easier way to configure proxies: https://addons.mozilla.org/mobile/addon/proxy-mobile/

I was suggesting that you check the desktop's about:config settings so that finding and setting the correct preferences in the mobile version's about:config is simpler.