I would not know of any simple way to install a previous version; in any case it would not be productive, as you would be missing all the new functionality, security and privacy improvements etc.

To free the memory, you may go and check the add-ons installed- Uninstall any entries you may not need and then check the memory still required, to see if it helped.

Let us know if it helped.

Regards

A quick comparison: Firefox 23 for Android uses 153MB of memory after a few minutes of use with 1 tab open, while Chrome for Android uses 51MB of data after a few minutes of use with 1 tab open.

For phones with 2GB of RAM, Firefox should be no problem, for those with a mere 1GB on phones using bloatware (Touchwiz, Sense, etc.), Firefox doesn't seem like a good option. At any rate, I run out of RAM fast when using Firefox for Android. (I have only Adblock installed as an extension.)