This is partly a technical debate, and partly subjective - ie driven by non-factual opinion.
Opera is a very secure browser, arguably the MOST secure. But there are some pages that it renders badly that Firefox renders correctly. Large Flash animations can be slow to render in Opera when scrolling the page up and down. So - Opera is a good, tight browser with well locked-down security, but looks a bit dull compared to Fox.
Seamonkey is a Mozilla browser, just like Firefox, but not so well maintained and supported as Firefox. It's quite good generally, and more secure than IE.
Firefox has occasional vulnerabilities discovered, but these are always patched VERY quickly by Mozilla, and workarounds published on the Mozilla web site. It also has a HUGE number of add-ons and plug-ins, the most famous of which is NoScript, which gives superb protection against scripting attacks both known and unknown, and has a customizable white list. NoScript does hamper the working of sites like eBay, but it's easy to switch off. Fox is well designed and the interface is tidy and logical, and very fast and intuitive to use.
IE is sort of OK, but Microsoft are very slow to admit security problems, and holes can go unpatched for WEEKS. That on its own is a good enough reason not to use it. The interface is badly designed, with tools and buttons placed in crazy locations. People changing from IE6 tend to hate IE7, and end up getting Firefox.
Tertiary - is that the word you were thinking of?