The reason I'm even mentioning it is because the issue doesn't occur in the PaleMoon release or Basilisk. By the way, it started (I think) with the next-to-last update of the unstable version. And, yes, I know that it's an "unstable" version.
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Anyway, what I'm trying to achieve is for the items in my Menu Bar to be centered.
I hope these three screenshots will illustrate what's happening in the unstable version.

In the top one, you can see that to the left of the Menu Bar, I have numerous "Spaces" and to the right a single Flexible Space. You can also see the four "text buttons" installed by some of the extensions I use. (And yes, I'm one of the only people in the Universe who uses PaleMoon's "Text Only" option.)
In the middle one, you can see how it should be. And when I place the flexible space to the right and then close the "Customize Browser" panel. It works fine for the rest of my session.
The "problem" occurs when I close and reopen the browser. In the bottom image, you see what happens: Somehow, of its own accord, the flexible space "hops" to the left of the four text buttons in question.
Of course I realize that my browser desktop is pretty weird and that this might be related to the full theme I use (Red Cats, Green Flavor).
Thanks for reading this. I'm looking forward to reading your observations.
P.S. I wonder if this is related to the issue mentioned in the thread "Appearance addon not displayed properly" by cartel (29 Jun 2018, 00:46)