Firefox

Is there a thing like good extensions and bad extensions on firefox. I mean, I don't want to know about privacy and security of extensions right now, but does some extensions slow down the performance of firefox more than other extensions?

Also, how to distinguish these extensions from others? Does size of the extension matter? i.e., if the size of the extension is higher does that mean it's eating more resources or will eat more resources.

Does a good extension run on all pages or only on the pages it's needed? Is there a better way to handle extensions? Maybe disable some extensions which are not in use, idk!