[yclept]

It is an http header: X-UIDH added to http requests

[andymurd]

It occurs to me that a mischievous person could easily write a Firefox plugin to (over)write that header with random garbage. If enough people used the plugin, it would render Verizon's data useless.

[clort]

except that the ISP is adding this header. They could (or do already?) just replace your header with their own..

[yclept]

Yep, and Verizon does. They will overwrite your header.