fix: Reset browser.link.open_newwindow on desktop Firefox too.

[emilio]

This confused me quite a lot, see
https://bugzilla.mozilla.org/show_bug.cgi?id=1598947

Changing the default behavior of target="_blank" is quite arbitrary and
unexpected, IMO.

[coveralls]

Coverage remained the same at 100.0% when pulling 19051e1 on emilio:master into 3c850ae on mozilla:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 30894fe on emilio:master into 3c850ae on mozilla:master.

[emilio]

npm run audit-deps is failing, but that doesn't seem related to my change.

[Rob--W]

Thanks. This has annoyed me for a while too.

If you'd like to use this feature before the new version is published, create ~/.web-ext-config.js with the following content:

module.exports = {
  run: {
    pref: [
      // https://github.com/mozilla/web-ext/pull/1766
      "browser.link.open_newwindow=3",
    ],
  }
};