One of the great things about Firefox is the number of themes you can apply to it. This includes festive themes and of course, Halloween.
Halloween Theme for Firefox gives Firefox a spooky look and adds such things as a cobweb to the URL bar and replaces many Firefox icons with scary things such as pumpkins, witches, skulls, ghosts and black cats. Unless you know what each button corresponds to by virtue of it's position in Firefox, this can become a bit confusing however.
You get a few other nice little touches too such as a ghost image when pages are loading and even your preferences have been given a spooky look. Bookmarks are also tagged with bats symbols next to them. The only real problems is recognition of what button's correspond to what function and the spooky illustrations and designs can obscure Firefox a bit.
If you're a Firefox user and want something a little more festive this Halloween while browsing, Halloween Theme is a good start.