Space bar scroll down doesn't work as expected in Firefox

Created by: macraig

Issue reported by a user from customer https://app.hubspot.com/contacts/2762526/company/768958891

Steps to reproduce:

1. Open any file on Sourcegraph Web with Firefox, example: https://sourcegraph.com/gitlab.com/sourcegraph/sourcegraph/-/blob/monitoring/definitions/frontend.go
2. Click on a blank line and space bar to scroll down.

Expected behavior:

Pressing space bar should trigger scroll down in file.

Actual behavior:

Pressing space bar doesn't do anything.