Why would you want to disable js on private tabs, you know disabling js breaks lots of sites, right

If I want scripts to work, I'll enter via ordinary tab. It's easy and convenient. But for paranoid strict level of anonymity I want scripts to be disabled. Tuning up whitelists or blacklists isn't so flexible and convenient solution.

You could also try a addon such as no script.

https://addons.mozilla.org/en-us/firefox/addon/noscript/

Thank you, I took Android version 3.5 from http://noscript.net/nsa/ It's very inconvenient and unstable :-( Better I'll manually edit JavaScript.enabled in about:config every time when needed.

More about anonymity. There is such a thing, as "fingerprinting", which can break the anonymity completely. Here is an article about 2010-year plans of Mozilla team to protect Firefox from fingerprinting: https://blog.torproject.org/blog/firefox-private-browsing-mode-torbutton-and-fingerprinting

But until now I don't see a way to protect Android Firefox from fingerprinting better, than simply turning off JavaScript.enabled at all.