"Enable error reporting" with Sentry breaks Firefox extension

Created by: dadlerj

Reported by a user:

Great work on Sourcegraph! I use it daily when conducting code reviews on GitHub, through the Chrome extension. However, AFAICT the Firefox extension doesn’t currently work. I think it used to, but when I hover over e.g. function calls in Firefox, Sourcegraph doesn’t respond. The extension is enabled and doesn’t report any problems - I’ve got version 20.1.5.708 and Firefox version 72.0.1 (64-bit) on OS X Catalina.