FIREFOX:
I actually quite like it, the grid goes for your eyes after a while though, and it seems a bit unaligned at the top.
IE:
Quite nice still, except that the top is still unaligned.
I like the sliding menus. Did you get it from Dynamic drive? I cannot find any comment saying it's from there.
I'm sure it would be possible to do the same thing in flash.
You could just use a trial of Swish or something.
ON THE FRONT PAGE:
I suggest you change the background of the table in the middle to be the same as the GIF.
I like the XP-esque stylings, looks nice.
EDIT:
I also like the box that comes up when you click on the maps, I see where your coming from when trying to make it look like hammer.
You should change it to a pop-up or something, so it can be moved about.
You could make a smart-edit style menu, with the different tabs for information.
You've obviously put a lot of work into this, and if it wasn't for the glitches it would be a pretty neat site.
Just drop the javascript menus, perhaps make the site different elements of the hammer editor? For example. click on the texture application box for about us, or configuration for credits etc.
Don't make it mystery meat though...