An experiment with opening mobile views of pages in the sidebar
Use npm install, then npm start.
Check out the repository. Go to about:debugging in Firefox, and select Load Temporary Add-on. Select a file in the addon/ directory.
Or: install web-ext (like npm i -g web-ext) and run web-ext run -s addon/ --browser-console -f nightly
This adds a context menu item: Open in sidebar or Open link in sidebar. Select that, and the sidebar will be opened with a mobile view of the page.
Anthony_f's Sidebar for Google Search inspired this add-on's approach.