STR: 1. request a very long web page ex1. http://searchfox.org/mozilla-central/source/db/sqlite3/src/sqlite3.c ex2. https://dxr.mozilla.org/mozilla-central/source/db/sqlite3/src/sqlite3.c 2. since sqlite3.c has ~197k lines, we shall see nothing but the loading sign for this tab 3. after loading for a while (~10-15sec), try open a new tab for any web page Expect: New tab should work fine Actual: New tab is keeping loading like forever, and the whole content process is not responding eventually. You can still open/close new tab, but can't load/render any web pages anymore. I first noticed this on Developer Edition, then confirmed that this can be reproduced on Nightly as well.

this work should help, but generally this is expected behavior with our current configuration.

I can reproduce it on Mac OS X 10.10 on Nightly 51.0a1 (2016-09-05). What is the right component for this, Core: JavaScript Engine?

As Jim said in comment 1, while the long page loads from ex.1 or ex.2 the content process is busy handling that page, therefore while that is processed there is nothing that can be done for your followup pages. I know that there are plans for having parallel processing implemented for content: check https://blog.mozilla.org/futurereleases/2016/08/02/whats-next-for-multi-process-firefox/ In order to move this bug further, I'm assigning this to its rightful component.