Firefox's zoom functionality comes really handy in numerous scenarios, however if you are used to work with many tabs of the same website or domain, Firefox will think you want to zoom all those tabs equally using the same zoom measure. Personally I find this annoying. You can change this behavior by entering the about:config command in the address bar, and then filter the preference list by typing the following keyword: browser.zoom.siteSpecific By default this is set to True. Change it to False by double-clicking the item. This will prevent the browser from automatically zooming in all tabs from the same website.