have windows xp---1.46ghz -256mb ram can I safley download 3.6

Here is the system requirements page for Firefox 3.5 & 3.6.
http://www.mozilla.com/en-US/firefox/system-requirements.html

IMO, you could use a lot more RAM - 256MB is marginal and that will limit the number of tabs / windows you can have open at the same time.

As the-edmeister says, you could use more RAM, but generally you should be fine. Firefox 3.6 will perform about as well as Firefox 3.0 on your computer.