It should be noted that this extension strips ETag headers from all responses by default, which can break sites in surprising ways. As a developer of a web application that relies on ETag headers for vital functionality, I see not-infrequent support inquiries from ClearURLs users who don't understand the technical ramifications of this feature - nor do they understand why so many of the websites they use are so broken.
Have you considered using something other than ETag for your use case? It seems like ETag been compromised by trackers, and unfortunately this is why we can't have nice things.
We use the ETag header to make use of browser caching - not just for performance, but as a component of offline support. Yes, we could add an additional header with the same information to work around this specific extension for application-specific functionality using it, but that would leave the browser-based features broken.
While the ETag header may have been usable for cross site tracking at some point in the past [1], browser caches are isolated per-origin in Firefox, so there's no longer a cross-site tracking concern. That leaves it usable to identify you across sessions only in a first-party context, just like cookies, IP addresses (to a lesser extent), the Last-Modified header, and any number of other identification techniques ClearURLs doesn't block.
[1] I'd be interested to see any credible evidence of ETag headers being used for tracking in the wild - I've only seen theorizing that it _could_ be used as such, prior to cache isolation being implemented in Firefox and Chrome.
> ETags can be used to track unique users, as HTTP cookies are increasingly being deleted by privacy-aware users. In July 2011, Ashkan Soltani and a team of researchers at UC Berkeley reported that a number of websites, including Hulu, were using ETags for tracking purposes. Hulu and KISSmetrics have both ceased "respawning" as of 29 July 2011, as KISSmetrics and over 20 of its clients are facing a class-action lawsuit over the use of "undeletable" tracking cookies partially involving the use of ETags.
It appears that there have been at least a few cases of this in the wild.
The main distinction (at least to me) between ETag and the other tracking methods you mention is that ETag doesn't appear to be easily clearable by a user (although that sounds like something browsers should fix if they haven't already).
It's unfortunate that features like this end up getting co-opted by trackers, which leads to breaking legitimate use cases like your app in the process.
That's certainly credible evidence for past use I overlooked, though it remains unlikely to be useful with the advent of per-origin cache isolation.
The Last-Modified header can be used in exactly the same way, and isn't blocked by this extension, which harkens back to my original point: this is an extension that appears to see significant use by non-technical users, yet it breaks a browser feature by default. There are plenty of other methods of identifying a unique user that it doesn't prevent, so this seems like a pretty unexpected feature users should take note of.
