hi, firefox is a multi-process application and particular tasks are put into separate instances. this should not slow down your pc, on the contrary it makes sure that things don't block up and it improves stability and security: https://blog.mozilla.org/futurereleases/2016/08/02/whats-next-for-multi-process-firefox/

if you have a particular problem with slowness, please expand on it and provide a bit more detail, so we can look into that. thanks

Hi, you are shutting down processes that Firefox is using. Firefox since before 57 but officially in version 57 became a Multiprocessor Support Browser. So things run in the own process such as the Sandbox such as : Flash, Sound. video is kept separate from the Firefox operating system. Other things that use processes would be your add-ons/Extensions, multiple tabs/windows pen. Version 58 I think Firefox also became Multi-Thread just like your computer chip.

So things are doing what they are suppose to be doing. What do the total processes on average add up to for ram ? Are you experiencing high CPU usage ? Are you having issues with lagging or locking up or  ? Do you have the ram to spare as unused ram is wasted ram ? Multi-processing can be turned down or up and it can be disabled but this is usually only done in extreme cases of low ram and crashing.

https://support.mozilla.org/en-US/kb/performance-settings 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, if things are working fine do nothing here as it has been turned on for years.

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