Prompt users to open HTML downloads in Tor Browser

Not sure how off into the weeds this is, but in testing tpo/applications/tor-browser!696 (merged) when opening the downloaded poc.html file, it opened the file in my system Firefox 115 install in a default/new profile. Verified my HOME is set correctly via the start-tor-browser script by inspecting /proc/$pid/environ

I have not tested other platforms.