TBB zooms text when resizing browser window

I've been experiencing an issue when all pages are zoomed fairly significantly after resizing the browser window.

It happens fairly frequently (but not all the time). To reproduce: start TBB normally, load a few tabs, and then expand the window beyond the default (say 1200px). Now all the tabs are zoomed in. Even if you hit command+0 (to reset the zoom), other tabs you open will still be zoomed. Navigating to new pages will also be zoomed.

I also tried to install a few zoom default extensions to no avail. I recently set the "privacy.resistFingerprinting" preference in about:config to false and that seems to be solved the problem.

Using TBB 6.5a1 on OS X 10.11.5