Current Firefox releases have multi-process windows enabled. You may see three or more Firefox processes running.

• one process for the main Firefox thread (user interface)
• one or more processes for content threads set via "Options/Preferences -> General -> Performance" (dom.ipc.processCount)
• one process for the compositor thread (Windows)

You can try to reduce the number of content processes to see if that helps.

See also:

• Options/Preferences -> General: Performance
  remove checkmark: [ ] "Use recommended performance settings"
• https://support.mozilla.org/en-US/kb/performance-settings

and why it happens?how can reduce the number?

It happens to maximize the browser speed, it's the internal behavior you cannot change it. For more information about multiprocess, please check this page

to maximize browser speed?seems contradictory to me.more process mean slow speed and more core of cpu

Current Firefox releases have multi-process windows enabled. You may see three or more Firefox processes running.

• one process for the main Firefox thread (user interface)
• one or more processes for content threads set via "Options/Preferences -> General -> Performance" (dom.ipc.processCount)
• one process for the compositor thread (Windows)

You can look in "Help -> Troubleshooting Information" (about:support) to see the current multi-process state.

• "Help -> Troubleshooting Information" -> "Application Basics":
  Multiprocess Windows
  Web Content Processes

See also:

• https://www.ghacks.net/2016/07/22/multi-process-firefox/
• https://www.ghacks.net/2016/11/11/firefox-52-nightly-quantum-compositor-has-landed/
• https://hacks.mozilla.org/2017/04/firefox-53-quantum-compositor-compact-themes-css-masks-and-more/

yes but how can reduce the number of process?as you told me co-rel? also is a ability for current version release.you mean generally v57 and above?or for a specific subversion like 57.0.3 and above?

57+ Multi-Processor ver 58 brings Multi-Thread Processing also. 56 down 4 versions has e10's turned on the precursor to turning it all on.

go to the Firefox 3 Bar Menu --> Options --> Privacy & Security panel and under Permissions check (put a tick in the box) the setting to Prevent Accessibility Services from accessing your browser.

Monitor in Task Manager. Should stay below 2 gigs. If excessive then do below and monitor and change if need to...Test and go up again , go back down, it will eventually get a zone you and it can live with.

Go to the 3 Bar Menu then Options --> General --> Performance and untick everything. change the recommended size lower then see how it runs. Note: 1 = No Multiprocessor = slow again. Try 2 Restart Firefox after making these changes please.

Note : Hardware Acceleration is for Video Card, Monitor to see if remain off or to turn back on.

Please let us know if this solved your issue or if need further assistance.

ok i will try.thanks

I fail to see those multiple instances of FF Browsers and I have Windows 10x64 Pro FCU with FF57.0.4x64 installed? Even in my Task Manager only shows one FF opening and I have 9 tabs open in it right now?