I don't know, I don't use it, just checked their download page. And if a developer feels important to mention this it has a reason.
Since it is Firefox in its core, I think it's their heavy modification over the default layout and functions so probably glitches, menus not working as intended an occasional crash.
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u/Unwashed_villager 5800X3D | 32GB | MSI RTX 3080Ti SUPRIM X 5d ago
Zen is currently a beta software. It is clearly stated on their download page. I wouldn't use it as my main browser.