Export function outdated, won't work for Addons Marketplace

The .xpi file still uses manifest v2, which is no longer accepted. This causes errors like “incomplete theme”. Mozilla hasn't updated this export feature even though v3 has been required for a while.

There are some problems:

- It can't change context menu color.
- Exported themes can't be installed, Firefox says it is broken.

A workaround for sidebar text contrast issues when using dark theme.
:root {
color-scheme: light dark !important;
}

It doesn't play very nice with Vertical tabs, most presets are obv broken and making one from scratch is a real chore without previews.

Still does exactly what was on the tin at release though. No notes with conventional tabs. It's just a real shame.