I've called the big guys to help you. Good luck.

Why do you want to downgrade? Is there a problem?

There is no such Firefox browser version as 2.1.1

The only secure versions of Firefox currently are Firefox 40.0.3 and the 38.2.1 ESR.

Every earlier versions are potentially vulnerable due to have having the security fixes since. https://www.mozilla.org/security/known-vulnerabilities/

Since you are using Windows there is the option of using a old vulnerable version by way of portable Firefox for the one thing you need a older version for. The portable Firefox will not touch your Firefox 40.0.3 install and Profile used with it.