STR:

1. Make sure profile does not already have a chrome_debugger_profile folder.
2. Set gfx.webrender.all = true and restart.
3. Open Browser Toolbox for the first time.
4. Set gfx.webrender.software = true and restart.
5. Open Browser Toolbox.

Expected:

Browser Toolbox uses Software WebRender like the browser.

Actual:

Browser Toolbox still using WebRender, I know this because it has visual artifacts that only occur for me with Webrender (Bug 1679681).

Deleting chrome_debugger_profile folder fixes it.

It seems like a current design of Browser Toolbox. It keeps same pref after chrome_debugger_profile folder is created. Browser Toolbox has a capability to update prefs during launching it. But it seems to be used only for testing.

• https://searchfox.org/mozilla-central/rev/d504494eab038fd1bbc82eb4191bb334b4de2c8d/devtools/client/framework/browser-toolbox/Launcher.jsm#188
• https://searchfox.org/mozilla-central/rev/d504494eab038fd1bbc82eb4191bb334b4de2c8d/devtools/client/menus.js#92
• https://searchfox.org/mozilla-central/rev/d504494eab038fd1bbc82eb4191bb334b4de2c8d/devtools/client/framework/browser-toolbox/test/helpers-browser-toolbox.js#63

With the patch, prefs of Browser toolbox was updated during launching it.

From comment 1 and comment 2, it seems like a problem of Browser toolbox.

The product::component has been changed since the backlog priority was decided, so we're resetting it.
For more information, please visit auto_nag documentation.