What is the oldest version of Firefox that willl load new websites?

I'm running XP Pro SP3 on an older laptop T43 Lenovo.
I upgraded from Firefox 28 or 29 (something like that), to Firefox 47.
This new FF is not working well. It seems like it gets saturated after
it's used for a half hour or so. (Depends a lot of the type of website I
am running). Then FF tends to just freezeup. Often times it will just
hang for 5 or 10 minutes. Other times it is permanently frozen so i have
to do a 3 finger salute to close it.

To fix it, I have to close FF, then clear the FF cache, and close it
once more. Than it works for another half hour or so before doing this
**** again.

I stick with FF because I use the video download addons more than
anything else. To view Youtube videos, I have to go to a public WIFI at
the library or a restaurant. It's no fun watching them in those places
because I cant play the sound. So I just download them and watch them
later when I am home. Otherwise Id try other browsers, even though there
dont seem to be many choices Since Windows Explorer is way outdated, and
I refuse to use Chrome at let Google track me.

Anyhow, I want to downgrade FF as much as possible and still be able to
use current websites.