There is a feature that interferes with Firefox remembering your preferred zoom level for the new tab page. Here's where you can switch it:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste newtab and pause while the list is filtered

(3) Double-click the browser.newtab.preload preference to switch its value from true to false

After the change, if you have any "new tab" pages open, close them, and open a new one, change the zoom setting, and now it should be remembered.

donnorton said

When I click on the three parell lines on the right top I decrease % so more tiles show but when I go to another site and then want to open another tab the 100% shows up how do I change this and it will not change back to 100%

Thanks for the easy to follow changes you suggested works great