Can you show me a citation of this? Doesn't sound correct to me, and I am pretty sure I have heard them say that it is safe to do. One of the browsers will be configured for Tor and the other will not, so it shouldn't pass to Tor. Also unless you are accessing the site through Tor it will be (well, in practice) impossible for it to send data to Tor in reply to you, since it will have your IP address and not an exit nodes IP address. So the issue would need to be poor isolation between browsers, but I have not heard of this being a problem and have in fact heard the opposite (that it is not a problem). I think Tor button will isolate cookies well enough even if the browser has some issue (at least if you have it configured to isolate Tor cookies). Anyway maybe it should be avoided, but I have not seen any examples (with technical details) of how it could lead to problems. I can imagine a problem arising if there is not isolation between the browsers, but I think Torbutton will come to the rescue in these cases. Should probably wait for a more definitive answer before doing it though because I am not 100% certain and it is always better safe than sorry. this thread in Tor dev mailing list from 2008 indicates to me that it is probably safe to run both simultaneously, particularly since now the browser bundle does allow for it http://archives.seul.org/or/dev/Dec-2008/msg00027.html