Without activity stream, the page is opened directly at the right zoom level. With activity stream, you can sometimes notice the elements of the page being rendered small and then big.

Comment on attachment 8900144 [details] Bug 1383875 - When the zoom level is not 100%, about:newtab is loaded at 100% first and then zoomed in. https://reviewboard.mozilla.org/r/171538/#review177020

Pushed by edilee@gmail.com: https://hg.mozilla.org/integration/autoland/rev/3951a3435442 When the zoom level is not 100%, about:newtab is loaded at 100% first and then zoomed in. r=adw

This issue is still reproducible, but only on the first opened "New Tab" after the zoom page level was changed. The page is first displayed at the previous zoom level and then it quickly changes to the modified zoom level. I was able to reproduce this issue using latest Nightly 57.0a1 Build ID 20170918220054, on Windows 10 x64, Mac 10.12 and Ubuntu 14.04 x64.

We'll track the "first opened" issue in a separate bug.

Managed to reproduce the initial issue on 56.0a1 (2017-07-29). Aside from the issue mentioned in comment 5 and comment 6, I can confirm that 57.0b6 build1 (20171005195903) build is verified fixed, using Windows 10 x64, Mac OS X 10.11.6 and Ubuntu 16.04 x64.