Firefox PDF Viewer vs. Chrome PDF Viewer
Firefox PDF Viewer renders PDFs like a web pages, so you see faster loading PDFs where it utilizes only few resources. And here necessity to use third-party plugins in browser that consume more resources is avoided.
Both PDF Viewers of Chrome and Firefox were enabled by default in browsers. That’s why when you try to open a PDF via a link in web page in both of these browsers PDF Viewers will take over third-party PDF plugins. That can be annoying, but this can be easily solved by disabling Firefox PDF Viewer and Chrome PDF Viewer so that third-party desktop PDF Reader’s plugins installed can load PDFs inline in browser.