As mentioned earlier, if I had a Windows machine, I wouldn't be using Safari on it.
Now, having had macs since they first came out, I think I have enough experience to speak on em.
If you like plug-ins and extensions, then Firefox is definitely the way to go. I did for a year. Hell, there's even an extension to make it look like Safari!
But, after using it for a year, I noticed it was slower and less stable than Safari, and I've stuck with Safari ever since.
If there are "weaknesses and vulnerabilities" in Safari, I'd like to hear about them.