The Web Inspector does not know that an TIFF image with Content-Type: image/tiff is an image and decodes it as text. This leads to a very high memory footprint. In my case it was an image with 2500x2000 pixel which needed more than 800MB memory.
Simply load http://plugindoc.mozdev.org/testpages/dscf1665.tiff and go to the inspector. In the resources you see it is decoded as text.
Tested it on actual Safari and Webkit from today!
I did a test of creating a small HTML file that has an <img> with the referenced URL as the source attribute. That works fine.
bweinstein has determined that the problem is caused when you load an image - any image - or any non HTML thing really - as your main URL in the browser. It's not actually a TIFF problem.
For instance, you'll see the same thing if you load the following URL in the location bar of the browser:
Created attachment 43097 [details]
Landed in r50905.