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How to trigger a child process in desktop Firefox

Firefox is now multi-process, and not just for the plugin-container process.  For example, there is now (present but disabled in Firefox 25, and likely to be released in Firefox 27) a separate process that is used to update the thumbnails shown in a new tab.

As a result, sometimes you might want to test something in the presence of multiple processes.  Here’s how I’ve been doing it.

  • Delete the images in the thumbnails/ directory within the profile’s temporary directory.
    • On Linux it’s ~/.cache/mozilla/firefox/<profile>/thumbnails/.
    • On Mac it’s ~/Library/Caches/Firefox/Profiles/<profile>/thumbnails/.
    • On Windows it’s C:\Users\<username>\AppData\Local\Mozilla\Firefox\Profiles\<profile>\thumbnails\.
    • I’m not sure about Android.
  • Open about:newtab.  This triggers a thumbnails process.  It’ll live for about 60 seconds.  (If you’ve configured about:newtab to be blank rather than showing thumbnails, this might not work, though I’m not sure.)

Please let me know if there’s a better way!

(And if anyone can give me extra info on the things I’m not sure about, I’ll update the text above accordingly.  Thanks!)

12 replies on “How to trigger a child process in desktop Firefox”

Note that background thumbnailing is #ifndef RELEASE_BUILD (i.e. disabled on beta/release) for the moment, so not in 25. We plan to have it ride the 27 train to release.

You can also flip a pref to enable e10s in Nightly builds. browser.tabs.remote, I think. Ride the future!

That’s an interesting idea. It might actually work, as AWSY just opens and closes pages and doesn’t do anything too weird.

e10s might be able to handle it, but the memory reporting infrastructure and AWSY won’t — they would only measure the parent process, not any child processes. However, the good news is that I’m partway through a patch to fix the memory reporting infrastructure, and once that lands, it’ll be a small update to AWSY’s code to take advantage of it.

I don’t think AWSY results for e10s would be meaningful because many features are not working correctly and much optimization remains.

That said, browsing websites is fairly stable… if you disable plugins. Even Adblock Plus half-works. 🙂

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