Yes, it is safe. I'm currently on 5.10.3 since my PW4 jailbreak.
https://s3.amazonaws.com/firmwaredow..._v2_5.10.3.bin
All one has to do to get earlier firmware updates is change the version number in the url.
After updating, you'll need to reinstall the addons: python, KUAL, screensavers, etc.