2019-01-11 Original 4-star review:
Excellent add-on, but I am seeing a slight problem with Win7/SP1/x64 & FF64. When I click on the icon to go into the Tranquility mode, just before the mode is switched, the original page zoom gets reset to 100%. Naturally, when I close the Tranquility mode and go back to the original page, my preferred custom zoom factor is now 100%. I am not using any zoom-control add-ons - just the built-in/stock functionality of Firefox.
The zoom problem makes this extension completely unusable for me. The problem has not been fixed, and the developer has not commented on my bug report, so I am lowering my rating accordingly.