hi, you can set the number of processes that are spun up for web content within Firefox's performance settings. fully disabling multi-process mode woon't be a supported configuration going forward...

Hi if you are seeing 4-5 instances of Firefox running in Task Manager then it is doing what is suppose to do as a Multi-processor and Multi-thread Browser. If there is extensive use of ram such as 3gigs then there are ways to reduce this. But from what you are saying. It is doing just great as these are for Browser, Tabs, Extensions, Sandbox.

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