Produce MV2 build for Firefox instead

[arkon]

Based on diffs in https://github.com/LiveTL/HyperChat/pull/163

Context from @KentoNishi / @r2dev2:

• HyperChat is actually published in its MV3 form for both Chrome and Firefox stores
• Firefox UX is lacking for prompting for runtime host permissions, where the user needs to go and grant permissions manually in the extension settings (pending improvements: https://bugzilla.mozilla.org/show_bug.cgi?id=1839129)
• The MV2 branch is actually just for use with the MV2 build of LiveTL (which is published as MV3 for Chrome and MV2 for Firefox)

Still pending relevant logic differences so it actually handles the chat part properly in Firefox.