I've been using FF 3.5 since its June 30th launch with no problems to date.
All that is normally required is to install any new version over the old one.
Try Uninstalling your current non-working version of Firefox, download Firefox 3.5 again (I presume you are using IE or some other browser at present) and Install it. See:
Any add-on or extensions updates should mostly be available very quickly afterwards.