Thats very strange. I've been running 1.5RC3 here since last week. Smooth as can be. Did you uninstall your previous firefox installation and extensions first Stoneyface? Plus, skins are for winamp (the birds) not browsers. I find this version a considerable improvement over 1.0.7 in performance.
Note: anytime I "upgrade" firefox, I uninstall it completely, delete the program folder and profiles folder and start completely fresh. It seems to work much batter that way, as I had encountered bugs in tha past when I failed to do that. (Actually that habit goes back to when I used Mozilla and actually did use skins, on an upgrade it caused issues... haven't used skins since.)
I'll try 1.5 later today.
edit: P.S. I do not use Winamp since Nullsoft sold out to AOL with it, Winamp now phones home to AOL servers on every startup. Thankfully Nullsoft has not handed over control of NSIS to anyone yet... <crosses fingers>