Though I usually prefer free software, I don't have any problems using proprietary stuff if I think it's good. I had Firefox open for a couple of days and noticed that it was using 700M of memory. That's not a problem at all since I have 4G but it's also a lot of RAM to be in use for just one window with one tab open. The fact that Firefox gets sluggish after some time and needs to be restarted tells me that this isn't expected behaviour and is likely not due to caching for quick back/forward or whatever they claim is taking up the leaked memory.
Konqueror is ok but I'm not a huge fan of it, partly due to its kitchen-sink browser/file manager hybrid design. IMO the KDE folks should break out the file manager part, but I digress. I can't really put my finger on anything specific I dislike about Konqueror, it's just not for me. To my dismay it seems to be the snappiest browser on Linux.
The only other decent browser I know of (for Linux) is Opera so I found some quick instructions on the Ubuntu forums and shoehorned the x86 build of it into my amd64 installation. Everything went well, Flash works and all that stuff. Opera is not nearly as snappy as I like but it is still fairly pleasant to use, once you find a skin that fits into your desktop. For the record Firefox isn't snappy enough either. Apart from AdBlock I don't miss many extensions for every day browsing.
I'm not sure if I'm going to stick with it yet but I've been using it for 2 days and haven't really missed Firefox at all. Of course as soon as I do any development I need Firefox for Firebug and the Web Developer extension and such. I've yet to investigate development tools on Opera. I'm comfortable developing in Firefox already so why switch?
Man am I glad we're not in a Netscape/IE world anymore! If I open up my MacBook I can choose from at least 2 other browsers for every day browsing (Camino, Safari).