Firefox plug-ins save into diffent folder Mac
The /Library/Internet Plug-Ins folder contains plugins that can be used by any browser run from any account on your Mac. The /Users/reashedzamil/Library/Internet Plug-Ins folder contains plugins that can be used by any browser but only when it is run by you.

However a better way to check which plugins are actually being used by Firefox is to type about:plugins into the address bar in Firefox. It will list all the plugins loaded into Firefox and tell you where they are. You can also visit http://www.mozilla.org/plugincheck/ which will analyse your browser plugins and tell you which ones are out of date.

You will also find another folder called something like /Users/reashedzamil/Library/Application Support/Firefox/Profiles/RANDOMTEXT.default/extensions that contains plugins (also known as Extensions or Add-Ons) that will be run within Firefox only when it is run by you. However if you think that extensions are the cause of performance issues, the best way to remove them is to open Tools -> Add-Ons from the menubar and uninstall them from there. 

Another thing you can use to check for performance problems in Firefox is to type about:healthreport into the address bar.