I cannot get to collections without closing all of my open tabs.

Simply press the + sign and open a new tab.

For now I've uninstalled it. You say "simply" but it isn't intuitive! I'd really prefer to just use the previous UI which is far superior!

Ok, no problem, thank you for your feedback. Please feel free to try the new Firefox Preview browser again as we roll out new features and fixes.

You say "simply" but it isn't intuitive! I'd really prefer to just use the previous UI which is far superior!

Even though the look is slightly different, the behavior of the latest Firefox Preview is now pretty much consistent with 'the previous UI', a.k.a. with the classic Firefox for Android. Therefore I'm not sure what exactly your problem is.
What's still a mess is re-using an existing tab. With the previous UI, one just had to tap the URL bar, which did bring up a view with Top Sites, Bookmarks, and History, straight forward, and easy to access.
There is no way to do that with Firefox Preview. With Preview, when tapping the URL bar, it just brings up a View with a bunch of search engines, which is pretty much useless.
In order to get to Top Sites, you'd always have to open a new tab. And even there, easy access to Bookmarks and History is still lacking.

As this has been brought up multiple times in this forum before, by now I'm not convinced developers have understood that the current behavior wrt re-using existing tabs is bad, and inconsistent with current behavior of classic Firefox for Android.

For now I've uninstalled it.

Up to you. But you may be better off getting used to Preview, as it will replace classic Firefox for Android at some point in the not too distant future.

@christ1 Thank you for taking the time here. I know that for the product owners/designers, there's no one right answer.

Personally I preferred the previous version Firefox Beta/Preview UI over the classic Firefox UI, but the latest UI was jarring. When I'm forced to move to it some day, I will also find it jarring! But I'll eventually put up with it and get used to it. I know that's just one person's opinion, and you have to create a product with mass appeal.

Of course, keep in mind that the first and primary UI of the browser is critical to the users, and changes to it will be alarming! If I had a choice between the two, via options, I would take the first one, but I also only have a few really high-traffic items in my collections that I want super easy access to. Others may build extensive collections that they want to hide away but get to from time to time, and some will keep a large number of tabs open, so they don't want all that on the same screen as the collection. I understand. It just annoys me when I have to endure a change like this, but your job isn't to avoid annoying me, it's to arrive at a reasonable compromise for a large number of users.

Thanks again for your time and great work, and keep it up!

Thank you for taking the time to provide your feedback, we really do appreciate it.