Ad-blocking plugins are not as well developed as they are for Firefox. In addition, there are tab-browsing plugins for Firefox that put Safari to shame.
Which ads? I have pop-blocking turned on and have not had a pop-up ad in years. I don't mind the ads on sites... I recognize that is what pays for the site's bandwidth. I do object to ads popping up and blocking my view of the page I am going to... but I haven't seen one in such a long time I've forgotten how annoying they are.
Tabbed browsing has been part of Safari for a couple of years now... and with Safari 3, you can move tabs by dragging them, pull a tab off the tab bar and make it an independent window... or drag an existing browser window onto the tab bar of another window and make it a tab, and grab a tab from one window and move it to another... sets of tabs can be saved to be re-opened at a later time, bookmarks can be made for all current tabs with a single click... or all windows can be combined by a single click onto one window as tabs. What more can you want? What do the Firefox plugins do that is not on the list above?