Tested Tails 3.5, but unfortunately same problem exists with TOR. Also tested Tails 3.6, released March 13, same problem again! I've not tested Tails 3.6.1.
I spent some time on this and came up with a suitable workaround. Simply find a portable browser app for Linux and copy it into the Persistent Volume. Just run it from there and open up the MyEtherWallet index.html. I used the Firefox Appimage located here: https://www.linux-apps.com/p/1168996/
Works great! Hope this helps.