GlobalZoom Mozilla Addon

Zoom in and out with the zoom controls, and it will affect all tabs. Can use keyboard hotkeys as well. Can also hold down the 'Ctrl' key and use the scroll wheel on the mouse.

By default, when you use the zoom feature of the browser, it changes the zoom level on that website only. This add-on makes it so that it affects all.

Default Preferences
This add-on resets the browser.zoom.siteSpecific and the browser.zoom.updateBackgroundTabs preferences to the default values. This usually is not a problem, unless you had used non-default values here. On uninstall/disable of this addon, the values are not restored to the custom settings. They remain at default. You will have to restore it to your custom value if you had set it.

Android Support
This add-on does not work on Android. I tested it, and it seems tabs in Android have their own individual zoom. I need to research this more.

Credits
symu for Icon