hi, the recommendation is at least 512MB of RAM: https://www.mozilla.org/en-US/firefox/43.0.3/system-requirements/

all recent builds still support windows xp, but for security reasons probably the most recent one (that's firefox 43.0.1 at the moment, that auto-updates to 43.0.3 for xp users) should be used. https://www.mozilla.org/en-US/security/known-vulnerabilities/firefox/

you also might want to try to set firefox to never remember any history in the menu > options > privacy panel, which can make firefox a bit more lightweight on resources.

You could try the SeaMonkey as it tended to work better on a old system that had a P4 2.8 HT and 1GB RAM and Ati X850 XT 256mb, especially over time. Though I mainly used Linux on it with Xfce for desktop to be light.

http://www.seamonkey-project.org/

Downside is the SM 2.40 update still has not been released.