You're probably seeing the new Multi-process feature in Firefox. Multi-process Firefox codenamed "e10s". https://wiki.mozilla.org/Firefox/multiprocess ''Started with Firefox 48 for a very small percentage of users who had noi extensions installed; expanded in Firefox 49 to a slightly larger group of users; further expansion in Firefox 50. Automatically disabled when too many extensions installed are detected and when extensions that don't yet support e10s are detected.'' Extension compatibility for the most popular extensions which are hosted at AMO are listed here: http://arewee10syet.com/ To disable multiprocess: Type '''about:config''' into the address bar, press Enter, accept the warning. Out this in the Search field at the top '''browser.tabs.remote.autostart''' . Double-click that preference in the reduced listing that is below the Search field to toggle the 'Value' from '''''True''''' to '''''False'''''.