(edit 8-20)

Looks like using the sidebar isn't too viable right now with the API limitations
* no POST capability (no page injection for POST code, sidebar not considered a TAB)
* can't open sidebar from quick menu / toolbar search (only available to context menu searches)
* limited width unless user edits files in profile dir
* sidebar always opens on install for FF pre-62 (minor annoyance)

Maybe in the future when the API is a bit more robust I can look into it again.

(edit 8-19)
The web extensions API is limited in how sidebars can be opened. I can send search results to the sidebar, but I cannot OPEN the sidebar programmatically in some cases. This may effect usability.

(edit) I'm trying it out now. The sidebar width is limited but I've read it can be increased by making changes to userChrome.css . I can add the option to open search results in the sidebar under -- search actions -- without much additional code.

You want to open search results in the sidebar vs in a tab? Could you explain a bit more about how that would be useful to you?