Requests on mousedown aren’t made in Firefox #435

Prerequisites

• I verified that this is not a filter issue
  • Filter issues MUST be reported at filter issue tracker
• This is not a support issue or a question
  • Support issues and questions are handled at /r/uBlockOrigin
• I performed a cursory search of the issue tracker to avoid opening a duplicate issue
  • Your issue may already be reported.
• I tried to reproduce the issue when...
  • uBlock Origin is the only extension
  • uBlock Origin with default lists/settings
  • using a new, unmodified browser profile
• I am running the latest version of uBlock Origin
• I checked the documentation to understand that the issue I report is not a normal behavior

Description

instant.page allows to preload pages on mouseover, but the requests aren’t made on Firefox when hovering over links.

It works normally in Chrome.

A specific URL where the issue occurs

https://instant.page/

Steps to Reproduce

1. Go to https://instant.page/ with uBlock Origin
2. Open DevTools and its network tab
3. Hover over items in the menu (at the bottom of the site, or on the right for large displays)
4. See that no requests are made. If uBlock Origin is removed these requests are made

Expected behavior:

No requests are made.

Actual behavior:

Requests are made.

Your environment

• uBlock Origin version: 1.18.4
• Browser Name and version: Firefox 65.0.1 (64 bits)
• Operating System and version: Windows 10