That's showing about 400MB RAM usage, and about 800MB address space. But address space includes mmapped files and reserved address space that is not actually backed by memory; it only matters for purposes of running out of a 32-bit process's 4GB address space.
So OK, 400MB memory usage. Of this, about 260MB was actually allocated by the browser (see "explicit"; the rest seems to be things the OS is putting into the process memory space space (e.g. the code of the browser, the code of the libraries the browser links to, etc).
Of this 260MB, looks like about 70MB is RAM used by your extensions (17MB for adblock plus, 6MB for https-everywhere, etc). Another 30MB looks like it might be JS GC heap fragmentation from those extensions.
Another 40MB is the yahoo mail tab; almost all of this is the various JS gunk it's doing.
7MB is Wired.
About 6MB for Slashdot.
Another 5MB for about:addons, and about 15MB for the browser UI.
30MB unknown to about:memory.
16MB in-memory cache for the bookmarks and history databases.
10MB images.
7MB web workers used by ghostery.
That accounts for most of the memory listed as far as I can tell.