To reconfigure Tor Browser so you can access the local web interface, apply the following six steps.
TODO: Check with TBB series 7.0+ once it becomes stable. There are alpha series bug reports that it doesn't work https://forums.whonix.org/t/new-version-of-tbb-no-longer-accepts-foxyproxy-plugin/2565/32
cp /usr/share/usability-misc/tbb-foxyproxy/foxyproxy.xml /home/user/.tb/tor-browser/Browser/TorBrowser/Data/Browser/profile.default/
Step Two: Download and install the add-on:
Browser Settings ->
Search: "foxyproxy" ->
Install: FoxyProxy Standard
This is safe to do since all add-ons on Mozilla's servers are now signed and verified by FireFox 43+.
Step Three: Restart Tor Browser.
Note: Tor Browser will soon ship with sandboxing on an opt-in basis. Unfortunately the initial sandbox versions are incompatible with such configurations and must not be enabled.
To reverse this and restore the default Tor Browser fingerprint its best to use a VM snapshot taken before installing the add-on. Re-downloading Tor Browser is another option in case no snapshot was taken. Alternatively FoxyProxy can be removed via