Firefox is better for compatibility.
One web site developer has once said that Safari is hard to develop for because it doesn't support some things that many web sites require.
For instance, these don't work in Safari:
-chat in Gmail
-Yahoo! Mail Beta
-Windows Live Hotmail full version
However, that developer has also said that the Safari 3 Beta is much better in this respect and supports a lot more things.
So support and compatibility for Safari might improve in the future.
A little suggestion:
If you wish to have Firefox's compatibility
but want a more Mac OS X-like look and feel as well as speed,
try out Camino:
http://caminobrowser.org/
It is made in Cocoa so it runs faster than Firefox.
It uses the exact same rendering engine as Firefox, so sites are supposed to display
exactly the same as in Firefox (except Camino uses Aqua widgets in web pages for buttons, dropdown menus, etc.).
The only thing is that a lot of sites detect whether you have a compatible browser, and not rendering engine.
As such, for some sites, Firefox is allowed
but Camino is blocked (even though it would display the site the exact same way as Firefox).
The solution to this is to change the user agent:
Type about:config in the address bar,
then find and change
general.useragent.vendor to
Firefox
general.useragent.vendorSub to
2.0.0.4
Now sites will think that Camino is Firefox.
This way, you can have the best of both worlds.