https://support.mozilla.org/en-US/kb/update-firefox-latest-version '''49.0.2''' ----------------- (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 '''async''' and pause while the list is filtered (3) Double-click the '''dom.ipc.plugins.asyncdrawing.enabled''' preference to switch the value from true to false This might not take effect until the next time you exit Firefox and start it up again. ------------------------------ One of the headline changes in Firefox 48-49 is e10s, which separates the browser interface process from the page content process. The performance impact of this can vary a lot between systems, so it would be helpful to know whether you have this feature turned on. To check that, 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 you are using e10s:'' To help evaluate whether that feature is causing this problem, you could turn it off as follows: (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. Any difference?

I checked both about:config and about:support. Both the settings are as expected. Please see attached images.