Docs for installing Firefox extension could be much better

Created by: slimsag

I observed a user installing the Firefox extension.

• Landed on https://docs.sourcegraph.com/integration/browser_extension
• Clicked Install Sourcegraph for Firefox
• Immediately presented with "Requires access to all sites", accepted it
• Went to Go files on GitHub and got nothing.
  • Went to public Go files, clicked on things, hovered on things, got absolutely nothing.
  • Got GitHub find-references/j2d and wasn't sure if it was GitHub or Sourcegraph.
  • Questioned if he actually installed it, went to addons page in Firefox and saw error:

• "Let's just assume I'm trusting enough to assume this isn't a virus...." and eerily clicked trust anyway enable slider.

Even if we cannot fix these, we could easily make this process MUCH more obvious by linking users who click Install Sourcegraph for Firefox to a page that explains what is going to happen next and why they should bypass all of the extremely dangerous-looking warnings Firefox present to them