See http://www.apple.com/findouthow/mac/#appstartup

Thanks for the useful reply to my problem. That all makes sense but this has led to another problem, quite a bizzare one. On getting the reply I immediately looked around for my firefox application to put it in Login Items and I cant find it anywhere! Its not visible. I have a Firefox alias in the dock and the browser starts so it must be there but Ive hunted everywhere especially in Applications where I thought it should be and no search on Spotlight reveals it either. How can this be? Any suggestions where the application will be? Do I need to reinstall?

All solved now though some weirdness going on on my computer. Switched on this morning and not only no Firefox application but the Firefox alias which was there (and Firefox working) when I shut down last night had disappeared from the dock! Spooky. Anyway did a fresh download from Firefox site and everything is back to normal. Have put it in Login Items now and assume it will burst into life when I start up next time. Thanks, job done.