This is likely to be a duplicate but I couldn’t find an existing bug (must be using the wrong keywords, or this bug only affects myself). On Nightly on OSX for the past few days, opening new windows (normal and private browsing) and opening dropdowns from webextensions seems to be very CPU-heavy. This comes with a noticeable delay, around a full second or more. Here is a profile with two CPU spikes on the main thread: https://perfht.ml/2GPtueb 1. First spike is opening a new window with Cmd + N 2. Second spike a few seconds later is when opening the Gecko Profiler dropdown so I can stop and save the profile. I’m seeing this issue on every window opening, and all webextension dropdowns (seen with uBlock Origin, Multi-Account Containers). Other toolbar dropdowns (main menu, overflow menu, address bar overflow menu) are not affected.

I think the same problem prevented me from opening extension dropdowns from the overflow menu (the overflow menu dropdown would close, with the focus still on the button). Anyway, I’m glad I captured a profile because I cant reproduce anymore. What I did: - Test in Safe Mode: no noticeable issue - Test in normal mode, disabling a few extension: no noticeable issue - Re-enable extensions: everything still works normally

Correction: I had disabled gfx.webrender.all With gfx.webrender.all;true and a browser restart, the CPU spikes and noticeable delays when opening windows are back. Do you need more information from about:support (e.g. the Graphics part), or something else? Application Basics: Name: Firefox Version: 61.0a1 Build ID: 20180410220129 Update Channel: nightly User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:61.0) Gecko/20100101 Firefox/61.0 OS: Darwin 17.4.0 Multiprocess Windows: 1/1 (Enabled by default) Web Content Processes: 5/4 Enterprise Policies: Inactive Google Key: Found Mozilla Location Service Key: Found Safe Mode: false