Mozilla has been working on a brand-new version of Firefox for Android for more than a year already, and while we thought that the general design and navigation were already finalized for the time being, the company is more open to experimentation than we anticipated. It's currently testing a complete revamp of the tab switcher in the Nightly version of Firefox Preview, giving it a dedicated spot in a new bottom sheet.
Previously, tapping the tab switcher button would actually open the new tab page, complete with open tabs, collections, and top sites. Heading to the tab switcher in the most recent versions of the Nightly release will instead make a bottom sheet slide up, giving you a list of open tabs, no matter if you set up the address bar to sit at the top or bottom of the browser UI. To dismiss individual websites, you can swipe them to the sides or use the x button. The design makes it easier to access private tabs, too, as they live in the same slide-up sheet.
Left: Regular tabs. Middle: Private tabs. Right: New tab page now without open tabs.
To add all open tabs to a collection, head to the three-dot overflow menu on the top right. If you're looking for those or top sites, you'll have to tap the new tab button in the bottom right corner. This change probably also means that Mozilla has finally all but given up on its concept of sessions it originally wanted to introduce with the new version of Firefox, for better or worse.
At the moment, the feature is far from stable — I encountered a crash after playing around with the bottom sheet for less than a minute. If you'd still like to try the new tab switcher for yourself, download Firefox Preview Nightly from the Play Store or APK Mirror. Just keep in mind that you'll have a more stable experience on the regular Preview or Beta channel, which will likely soon receive the new tab switcher.