Downloading of binary files stalls at 0/0 bytes in Tor Browser based on ESR52 with e10s off

A user reported in #21340that downloading of zip file stalls reliably at 0 bytes with

[04-07 10:07:18] Torbutton INFO: External app requested
http channel Listener OnDataAvailable contract violation

Stalling happens as well for other binaries that need the external helper dialog, e.g. https://dist.torproject.org/torbrowser/6.5.1/TorBrowser-6.5.1-osx64_ar.dmg.

A workaround is to cancel the download and restart it on the about:downloads page.