Because the Firefox interface is depicted in XUL language. The advantage of XUL is good cross-platform compatibility, so that Firefox can be well used for windows/mac/linux. But the bad thing is that XUL is not efficient language, which is the main reason why Firefox starts feeling slow and the system resources occupy high.
If you care about speed and resources, try a third party/unofficial version (third party/unofficial builds). These versions for different platforms, at compile time to do a lot of optimization, relative to the official version, will be faster in speed.