Agree with sibling comment as someone who used Zen for many months, maybe as long as a year or two. It constantly breaks and often stays broken in small but fundamentally important ways, to the point that I just switched back to FF last week and am glad to be off the roller coaster. Before Zen I had tried Arc and left for a lot of the same reasons.

For all of the (valid) criticism against FF, it's still the best available browser that's not just an experiment IMHO.

Edit to add: part of the switch back is that FF now supports, to some degree, all the features I was using Zen for: vertical tabs (needs customization but works well enough), custom search "engines" (ie, shortcuts), split view, not-Chrome