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.

[wheelercj]

Firefox appears to plan on supporting MV2 indefinitely.

While some browsers are phasing out Manifest V2 entirely, Firefox is keeping it alongside Manifest V3.

— Mozilla’s approach to Manifest V3: What’s different and why it matters for extension users (February 25, 2025)

It will probably never be possible to make the Chrome and Firefox versions of Stardown exactly the same, so while making them more similar is nice and might make maintenance a little easier, it's not super important.

Stardown doesn't lose any features by switching to MV3 though.