Try doing a clean install of FireFox if you have it.
Everquest 2 uses Mozilla files for its in game browser and that xul.dll is a file from Mozilla distro.

Eq2 does use firefox for their in-game browers but it is a custom version i think. the xul.dll is dated 02/28/2007 .. 6,459,392 bytes file size. i think eq1 also uses it too as a search brings up a patch log with the same file size as that one.
being that you are missing a file i would be worried that you are missing others aswell. the best thing you can do is completely uninstall eq2 and make sure the eq2 directory is deleted as well. then reinstall and see if that helps i know you done that once already but just to make sure.
also alot of programs while they are installing puts stuff into your windows\temp folder check there while eq2 is installing and see if the file you are missing shows up and copy it from there.

I am sorry you are having trouble

Well, I tried copying all of the Firefox .dll files into the EverQuest2 dir and that just lead to a bunch of errors

I never said for you to copy firefox .dlls to your eq2 directory. Unless they are the correct version with the same date and file size they have no chance of working. Which is why I posted that info for you in case you came across them on your pc, you could see if the date and size matched.
I said that eq2 may put the files you are looking for in the windows/temp directory while it is installing. A lot of programs do but under normal conditions it should have copied it over to your install directory.
You said you reinstalled eq2 but how did you reinstall it? Uninstalling the program from your add/remove programs in control panel then making sure the eq2 install directory is deleted and reboot the pc.
then install from the dvd. The Rok and TSO box sets are the only disks that work with the emu.
then follow the post at http://www.eq2emulator.net/forums/viewtopic.php?t=98
but with those 2 box sets don’t patch it is not needed and the live client may not work with the emu.
if you done the above and eq2 did not load up at all then you most likely have a scratched disk or a hardware problem such as a hard drive starting to go out or memory starting to go out. that is a common problem for data loss as well of course malware or viruses.
If your disk is scratched or has crc errors then you have to patch to live and wait until there is a version of the emu that is compatible with live. Or take your copy back to the store and see if they will replace it some will and some will not, can’t hurt to try.
You can test your hard drive and memory using tools
For your hard drive the best tools for that is by going to the manufacture of your drive website and downloading their fitness test cd and follow instructions there.
as for memory memtest86 is a good program for that. http://www.memtest86.com/download.html download their iso and burn that to test out memory.
and there are free online scanners that will tell you if you have a virus or malware a good one is http://www.kaspersky.com/virusscanner
I hope these suggestions help.
Edited by bion.. The current live client dec 2 2008, 5280L works on TessEQ2.