Something keeps WebGLTexture alive and that ends up keeping the whole document alive.
The addon from Bug 726346 can be useful for debugging (restart after installation). After running cycle collector (maybe couple of times), "Find documents" shouldn't find any documents.
This is very possibly bug 795186: let's fix that first and see if this bug persists.
This is fixed by https://hg.mozilla.org/mozilla-central/rev/48502b61a63e , in my limited testing which consists in loading Nokia Maps, opening a new tab with about:memory, closing the Nokia Maps tab, and clicking GC and CC (but NOT 'minimize memory usage') a few times in about:memory and verifying that all webgl metrics disappear.
This seems to have been a regression from the port to WebIDL bindings as well --- just tested a nightly build from Sept. 17 and it didn't have the issue.