Firefox (53.xx) showed al multiple threads in the taskmanager. 55.xx doesn't. How can i get this back?

Hello every body, i'm using a pretty old PC. FF tended to get slow from time to time (maybe the plugins, maybe the excessive amount of tabs). So i was pretty happy with the fact, that FF (i guess it was 53.something) showed as multiple threads/tasks in the task manager. Yes, multiple. Most often 3, all named "Firefox", plus the plugin container tasks of coarse. 2 of the main tasks using only a small amount of ram (about 20 to 100mb), one with much more (most about 1gb when it started to slow down). I then was simply able to kill the task with the biggest memory usage, which leads to a "oh no, the webside has crashed" (or something like this. All loaded tabs got white with this message in red or black). Well, after simply reloading the tabs i was actually using, everything was fine (till FF would start slowing down again). Now, FF updated to 55.0.3, there is only one task in the task manager. But FF still gets really slow after using it some time (30min to couple of hours depending on websites an usage). But since there is only one task left, i have to kill this one and completely restart FF. Is there a way to get the multiple tasks back? Undo the update seems not to be supposed to work since it would reveal security holes ... Need help, don't want to use IE as a main browser. Thanks to everyone reading and thinking about this, Lukas Using: win7, 32bit, dualcore, ~ 3.7GB ram addons: NoSkript, addblock plus, Video downloadhelper