I'm one of those mad people who uses Seamonkey pretty consistently. We can even infuriate each other -- one guy still keeps his preferences from the old versions so I'm always getting files attached to emails which have as their names the original path from the net. Since I'm on Linux I can just feed the filenames into wget and fetch the originals rather than download the attachments. Nevertheless when it works the way you want it to I find it is more stable than Firefox (even though a firefox developer told me a year ago that it is literally the same browser as firefox, with just the name changed and attached mail, address book, chat (chatzilla) and graphical web composing components like their ancestors Mozilla and Netscape). Maybe it's just that Netscape was my first web browser and I never liked IE. So it's the familiar interface. I will use several web browsers so I won't say I only use Seamonkey. I am not, though, despite the identities of the two programs, the only one who prefers it.
Firefox has improved with 3.6. In fact I would say it is sometimes worth using. Still a memory hog. I LIKE underpowered computers, frankly. I use Linux, so it crashes a lot on me and I don't respect it that much. Seamonkey SHOULD be worse but for some strange reason isn't.
Google Chrome is great. While I haven't used Apple Safari in a while, they share a back end, the way Seamonkey and Firefox do. It's called Webkit and it's based on the web browser for the KDE desktop -- konqueror. The guy behind the KDE project just got a medal from the German Government, and overall he deserves it. Konqueror isn't why. Its engine -- which has become webkit -- is one of the most efficient out there. Its user interface is cruel and inhumane punishment unless you are into Bondage and Discipline or are a busy developer who will put up with anything to get where you are going on the web quickly. Safari uses proprietary code for its user interface. Google Chrome uses code which calls on the Mozilla Libraries for its user interface. Thus on my Slackware systems, which in theory don't support Chrome at all, all I have had to do is create symbolic links to my Seamonkey Libraries where Chrome can find them and it works as smoothly as it does on this Gentoo box (and Gentoo does support it. I use its ebuilds to get it for Slackware). Safari and Google Chrome are different programs but they share their browser engine and it is great.
A word about IE and from the Linux user's point of view. The old joke on Slackware lists is "If you surf the net as root you may as well be running Windoze". Microsoft's insistence that applications -- and in particular internet applications -- run in administrative space rather than on dedicated user space as is the default elsewhere is probably the single most important factor in the bootstrapping of our multi-billion dollar malware industry. It's not that Ubuntu for example does such a great job on security it's that Windows does such a LOUSY job. And IE is an excellent example of why. In point of fact a year ago they made an add-on for Firefox -- which can't be uninstalled -- which makes it JUST as unsafe. Now that the bad guys have seen the money -- and gotten their hands on a HUGE chunk of it -- they will not go away, but LITERALLY using Microsoft technology which depends on "Trusted Sources" it is ridiculously easy to spoof rather than trusted actions DOES help them. So think carefully before you clikc on the blue E.