Tabbed browsing, a faster alternative to multi-window browsing.
When you open a link in a new tab, you can continue reading or clicking links in the original tab while the linked page loads.
Unlike new browser windows, new tabs open instantly.
You can quickly open a link in a new tab by Ctrl+clicking or middle-clicking the link.
You can "undo" accidentally closing a tab.
Control. Firefox blocks unrequested pop-ups and never allows web pages to create hard-to-close full-screen windows. Firefox gives you options to prevent web sites from resizing and focusing windows.
Security. Malicious web sites can never install spyware without your permission. Wank in peace.
Privacy. Everything you might want to clear in order to cover your tracks -- cookies, cache, download history, history -- is in one place in Options. Firefox even provides a "Clear All" button to clear all of this information quickly.
Extensibility. The Pornzilla bookmarklets and extensions can improve your porn surfing experience far beyond what any browser vendor would dare.
New windows open quickly.
When you download a file and Firefox asks you to select a download filename and location, it starts downloading the file while it waits for your response.
You can save video clips quickly by Alt+clicking the (thumbnail) link to the video clip. If you save video clips this way, Firefox won't even prompt you for a filename (unless you select "Ask me where to save every file" in the "Downloads" section of Firefox's Options).
You can control whether videos are displayed in the browser window or saved in the "Downloads" section of Firefox's options.
File > Save As... works for movies being displayed by the Quicktime plug-in.
All visited URLs are shown in location bar autocomplete, so you can quickly see which
galleries and which images you've seen within a given site.
While using Firefox, you won't accidentally set a porn image as your desktop wallpaper. (Firefox has a "set as wallpaper" feature, but it asks whether you want the image tiled or stretched, and the dialog has a "Cancel" button.)