Version 2.0 of my Force-TLS add-on for Firefox was released by the AMO editors on Tuesday, and in incorporates a few important changes: It supports the Strict-Transport-Security header introduced by PayPal, and also has an improved UI that lets you add/remove sites from the forced list. For more information see my Force-TLS web site.
On a similar topic, I've been working to actually implement Strict-Transport-Security in Firefox. The core functionality is in there, and if you want to play with some demo builds, grab a custom built Firefox and play. These builds don't yet enforce certificate integrity as the spec requires, but aside from that, they implement STS properly.
The built-in version performs an internal redirect to upgrade channels -- before any request hits the wire. This is an improvement over the way the HTTP protocol handler was hacked up by version 1 of Force-TLS, and doesn't suffer from any subtle bugs that may pop up due to mutating a channel's URI through an nsIContentPolicy. I'm not sure that add-ons can completely trigger the proper internal redirect, since not all of the HTTP channel code is exposed to scripts, and add-ons would need to replicate some of the functions compiled into the nsHttpChannel, opening up a possibility of obscure side-effects if the add-on gets out of sync with the binary's version of those functions.
Edit: The newest version of NoScript does channel redirecting through setting up a replacement channel in a really clever way -- pretty much the same as my patch. It replicates some of the internal-only code in nsHttpChannel, though, and it would need to get updated in NoScript if for some reason we change it in Firefox.