Remove the ability to toggle Torbutton
Torbutton has long since become unsafe to use in a toggle capacity due to the change in our plugin handling code for #3580 (closed), and probably due to new Firefox features like WebThreads. We've wanted to do a release that warns users not to use it if we don't detect Tor Browser underneath us (#3838 (closed)), but I feel guilty doing that without an autoupdater for Tor Browser and with the lack of the ability to ensure build integrity (#3688 (closed) or at the very least #5837 (moved)).
However, if we disable the ability toggle entirely (and also disable a couple other things - #5758 (closed)), it might not be the end of the world if you still insist on using Torbutton in Vanilla firefox. You'll stick out like a sore thumb and be way more linkable across different websites, but at least the proxy probably won't be bypassed.