I am using FF a lot now. The rendering speeds are just so much better than Safari. The skin disturbed me but a link earlier on in this thread referred to a Safari style theme which I have downloaded and it's pretty cool.
One irritating bug I've found is when you click the drop down icon on the back button a drop-down list appears of previous webpages, which remains in place even after selecting one of the pages. I have to ctrl-click in the window to remove it.
Also, unless I'm doing something wrong, if I want to open the url I've just entered in a new tab I need to press alt-click, but if I want to open one of the stored bookmarks in a new tab I need to press CMD-click. Having to remember two different modes is a bit annoying.
Overall I like it because its faster, and that's sufficient for me to make it my preferred choice of browser at the moment.
Vanilla
PS: I tried out Shiira but did find the default fish icons annoying. I personally liked the Bookmark drawer though, nice.