Hmm, it seems like an opportunity for the author of Tab Scissors to create a version for Firefox. The closest WebExtension-based extension I found in a quick search was an extension that moves all the tabs into separate windows: https://addons.mozilla.org/firefox/addon/tile-tabs-we/ The author might be able to add the feature to split the tabs into two groups the way you like. Among legacy extensions (destined to stop working in Firefox 57), this one seems to do part of the job with its ability to move multiple tabs to a new window, but may not have the side-by-side tiling feature: https://addons.mozilla.org/firefox/addon/multiple-tab-handler/