1. Hide the unnecessarily huge sidebar header (allowing addons to show their own arbitrary fully custom sidebars separate from the official sidebar system could also work)
2. Hide the default tab bar
I currently run a vertical tab setup with Sidebery that looks great with custom userchrome, but the release that kills userchrome will make it too much of a mess to bother with a redundant tab bar and ugly space stealing sidebar header.
Additionally, an “adaptive” theme that pulls colors from the OS like Sublime Text’s adaptive theme does would be greatly welcome. While it’s nice that Firefox comes with a dark theme (and that there are plenty of third party dark themes), it’s irritating that it’s always a different shade than the rest of the OS — both the background colors (which change dynamically with wallpaper-adaptive appearance enabled) and accent colors (which is user specified on both macOS and Windows and soon on Linux with GNOME too).