Firefox extension

[wolfspelz]

• [x] Fix CORS and frame-src issues.
• [x] Fix scrollbars and enlargement corner of text fields behave like pointer up events are swallowed.
• [x] Fix can't click links through transparent area of avatars.
• [x] Fix long click not working in Firefox touch mode.
• [x] Fix icon (already fixed in a16e1a5514eb120ce2234e9d43565b9593a01d14 but likely didn't make it into release)
• [x] Register in Firefox Appstore
• [x] Wait for Appstore
• [x] Remove "not available for Firefox" messages from weblin.io and add support for firefox store in landing page.