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I am running Firefox Quantum 63.0.3 (32-bit) on Ubuntu.

In output of htop command (sorted by Command name) I see the following

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I see 6 children process below the original firefox process. What do they correspond to? I have more than 6 tabs open in the browser.

  • The number of content processes will one day vary based on the number of content origins if Project Fission gets going, which is kind of like Site Isolation from Chrome. – dsstorefile1 Nov 26 '18 at 23:30
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Multiprocess Firefox is an architecture that splits the Firefox browser into two pieces - a single parent process that displays the browser UI, and child processes that display the actual web content. These are the processes you see in the htop output.

As documented under performance settings, you can control the number of child processes.

Content process limit: With Multiprocess Firefox, also called electrolysis or e10s , Firefox runs web content for all tabs separately from the main Firefox process for increased security and performance. Using multiple content processes can further increase performance and minimize the impact of content process crashes. You can modify this setting if multiprocess is enabled; however, having too many content processes can slow down your computer, and consequently, Firefox. The recommended maximum is four, but you can set up to seven.

Therefore, the total number of processes are controlled by the configuration, and is not directly related to the number of tabs in use. You can check the configuration for your system under Options > General > Performance.


As listed under the Process model for Firefox, Multiprocess Firefox also uses child processes to sandbox untrusted content. In addition to web content child process, Firefox also uses sandboxed child processes for the following on Linux:

  • Gecko Media Plugins (GMPs), used for media playback
  • File content process, used to load 'file://' URLs
  • WebExtension Process, used to load extensions built using the WebExtensions API.

On Firefox 63.0.3, the WebExtensions process is also loaded at startup, which is the extra process seen in the htop output. You can verify this by accessing the about:memory URL from Firefox, and the selecting 'Show Memory Reports' > 'Measure'. The main process and the child processes (including the WebExtensions child process) will be listed here, along with their respective PIDs.

  • Thanks. I had that Content Process Limit option set to 4 (default). Why then the htop showed 6 child processes? Also when I changed the content process limit to 1, now, after restarting browser I see 2 child processes (instead of expected 1). – xxx374562 Nov 26 '18 at 7:01
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    @RogerThat I have updated my answer with details on the extra Firefox process. – Haxiel Nov 26 '18 at 17:40

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