One of the headline changes in Firefox 48-49 is that some users have a new feature enabled (e10s), which separates the browser interface process from the page content process. The performance impact of this can vary a lot between systems, so could you check whether you have this feature turned on? Either: * "3-bar" menu button > "?" button > Troubleshooting Information * (menu bar) Help > Troubleshooting Information * type or paste about:support in the address bar and press Enter In the first table on the page, check the row for "Multiprocess Windows" and see whether the number on the left side of the fraction is greater than zero. If so, you are using e10s. If your Firefox is not performing well with e10s enabled, you could try turning it off here: (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 '''autos''' and pause while the list is filtered (3) Double-click the '''browser.tabs.remote.autostart.2''' preference to switch the value from true to false ''Note: the exact name of the preference may vary, but it will start with browser.tabs.remote.autostart'' At your next Firefox startup, it should run in the traditional way. Let us know if you notice any difference. (It's really a temporary workaround for diagnostic purposes, and not a solution.)