Native side panel support

[Pseudoryx]

Native support for the side panel feature. This feature is available in multiple browsers, including Firefox forks (Floorp, Zen), so Firefox is clearly capable of it.

Example and Reasoning

The current (and only known) user-made implementation is through the use of userChromeJS. 

• https://connect.mozilla.org/t5/discussions/second-sidebar-with-web-panels-for-firefox-userchrome-js/...
• The feature is so massively useful that it should be prioritized for native support. The user-made version is great but it has not been actively maintained for a while and leads to complex visual glitches in the latest Firefox version.

Why current organization tools cannot replace its utitlity

• New sidebar: some panels that can be launched from the sidebar button strip behave similarly to the proposed side panel, but they are limited in options (settings, bookmarks, passwords, etc). Transforming them to a proper side panel would require more item selection, such as pinning more toolbar items (including extensions) and any websites. Such iteration also requires decoupling the current sidebar from vertical tabs so both features don't compete for space. 
• Split view: while both methods allow viewing multiple websites within a window, opening a website on a side panel allows you to jump between tabs while keeping the panel site open.

[Jon]

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