Update the Firefox version of Stardown to manifest v3

[wheelercj]

With the release of Firefox v129 on 2024/8/6, Firefox's manifest v3 blue dot bug will be fixed.

After a little while (since there might be a bunch of Firefox users that won't update right away), it would be good to change Stardown's Firefox version to use manifest v3. This will make it more similar to the Chromium version.

Note that Firefox does not support service_worker in manifest v3, so Stardown's use of a background script will not need to--or be able to--change.