Allow the user to move the icon into and out of the "…" (overflow) menu in the address bar. Currently, this is only possible for the built-in icons, but not for WebExtensions.

This is e.g. useful for https://github.com/kkapsner/CanvasBlocker/issues/177

I don't really understand. The context menu entries on the page action items should let you add/remove the item from the location bar. This was fixed in bug 1395387 and follow-ups, and should be fixed in 58 and later (released earlier this week). Can you elaborate on what version of Firefox you're testing with, what add-on, and what you are/aren't seeing happen when trying to use these context menus, and how that does/doesn't match your expectations?

Ah okay, did not saw that. It's indeed what I want and I can confirm that it is fixed. Thanks.