The problem is that Firefox was the last well supported browser that allowed for power user features. With this change, Firefox targets the lowest common denom

The problem is that Firefox was the last well supported browser that allowed for power user features. With this change, Firefox targets the lowest common denominator just like every other major browser (tracing the footsteps of Chrome). In the progress, it too, is becoming just like Chrome. This whole saga begun back when Opera dropped support for their 12.x browser, and became just another Blink-based browser. At that point, the only way to get a usable browser (with vertical tabs, MRU tab switching, gestures) was Firefox with a number of addons (Tree Style Tab, Tab Mix Plus, Firegestures). One may argue that users like this should just use a fringe browser with such features built-in, but that's not viable because they're either unstable or lack timely security updates. You may also argue that you can get all those features, even in Firefox 57, but that's only true with caveats: Tree Style Tab still exists, but if you use either the history or bookmark sidebars, you better get ready to switch back and forth all the time (since only one sidebar can be active at a time). The about:config setting browser.ctrlTab.previews (now also exposed in preferences as "Ctrl+Tab cycles through tabs in recently used order") is mostly useless, since it is limited to the 6 most recent tabs (probably even fewer on lower resolution screens). There are several mouse gesture addons for Firefox 57, but none of them currently work on Linux/Mac. Even on Windows (and the others when they're fixed), gestures will work neither on internal pages nor on addons.mozilla.org.