STR: - Open a really simple page in a tab. In my case I only had one tab opened in my browser, on data:text/html,test - Open the browser toolbox - Switch to the Perf tool - Start a recording - Switch to the browser window and do something quick, like resizing the window a little bit - Switch back and end the recording - Wait! I don't think it used to be this way, because I've used the perf tool in the browser toolbox a few times in the past, and I don't remember it being this unresponsive. Now, it's basically unusable, it freezes for minutes (maybe forever, I haven't tried waiting mor than 5 minutes or so) and is unresponsive to user events. It's obviously performing some really intensive tasks that prevents the whole process from doing anything else like: - hovering over tabs in the browser toolbox doesn't even display them in dark grey like they normally do, - trying to move the toolbox window doesn't work, or seconds later, - trying to select an area of the timeline doesn't work. In fact, after a few minutes, nothing works for me anymore at all and I have to kill the browser toolbox. I tried using the perf tool in the *normal* toolbox too, and noticed that right after a recording was made, there's a bit of time when the tool is unresponsive. That's when it needs to load a lot of the data and draw the various graphs. But after a few seconds it works fine. This initial processing that takes only a few seconds in a tab with a short recording is probably what takes minutes in the browser toolbox. Although I'm relatively certain that it didn't use to be this way. Tested on the latest FF51 on a desktop PC with Windows 10. And a rather powerful one too: - Dell Precision T1700 - Intel Core i7 3.6GHz - 32GB RAM