Since there’s no good way to add such a plugin to a Tor Browser after it’s built, and since when it’s webextensions all the way down the old Firefox branding approach will no longer work, I think the only way to build an i2p browser that doesn’t risk accidentally telling people it’s a Tor Browser at this point is to create a modified rbm configuration(Edit: and some make targets, more-or-less. Probably a little other stuff.) with a re-branding plugin incorporated into the build procedure. I’m re-reading through the TBB hacking document and the rbm manpages and I think this is surprisingly simple(Well, it’s extremely well documented and much easier than it was the last time I tried to compile a modern web browser from source). We won’t be reproducible against the regular TBB, but we will be reproducible(Against our new configuration), have hardening and Tor-relevant fingerprint protections, and not risk misleading labeling or documentation. (on the bright side, for me at least, it means I will get Windows and OSX builds for free. Not Whonix-relevant, just trying to be positive)
Of course this is all moot if there’s a hidden about:config or application.ini or some other option anywhere capable of doing what we need. But I couldn’t find one.