After many years of using FFox, I just tried a Zen install on Linux. It did not turn out as well as I hoped.
I did not have FFoxesr installed in the way the OS would have installed it (though it was still in the user folder). This meant that Zen did/could not see my bookmarks, extensions or passwords ... and the options it offered didn't work out. (It wanted an HTML bookmarks file ... I had them saved as JSON ... and a 'CSV' (??) passwords file ... wherever that is ... and it found no extensions folder.) So, for starters, years of customizations had to be manually restored.
But, fair shake, I did manually re-install bookmarks AND a few extensions that had saved databases (e.g. UBO, NoScript, Block site). (It ignored the sub-folders in the JSON bookmarks folders, dumping all bookmarks into the top-levels.) And I had to re-create all the settings. (Most of which exist in the .mozilla folder on Linux ... easy to find.)
I played for an hour with what I put there (without a menu bar ... or a tab bar, all URIs are shoved together -by name- in a sidebar ... I did figure out how to see a bookmark bar). I could discern no -truly useful- advantages to it. None. That was not offset by some pretty cosmetics. So even if you do get all of your customizations past the one-size-fits-all install, for long-time FF users I see no substantial advantages to the Zen browser.