YesScript is NoScript's Antagonist
NoScript is a highly acclaimed Firefox security add-on that protects its user from scripts that are executed on websites. The approach is to block all scripts on a website unless the website gets whitelisted by the user. These whitelists can be temporary for the browsing session or permanently.
While that is certainly the best security approach it does require lots of work especially in the beginning as you will encounter sites that won't work without at least some whitelisting.
Most users on the other hand prefer simplicity and no user interaction and that's where YesScript comes into play. Its approach is the complete opposite of NoScript: YesScript allows all scripts on all websites unless they are blacklisted by the user.
The advantage of this method is that less user interaction is required. It does however undermine the security aspect because scripts will be executed normally as long as the website is not on the blacklist.
There are other differences. YesScript adds a single button to Firefox that you click on to enable or disallow scripts on the domain you are on. This means that either all or no scripts are allowed to run on it which is different from NoScript's behavior that allows to enable scripts individually.
NoScript in addition to this ships with a set of additional security features that improve the overeall security of the browser further.
It comes down to an evaluation of the advantages and disadvantages of both methods. NoScript provides enhanced security while YesScript less work and vice versa. Installing YesScript from a security standpoint does not make that much sense but it is quite capable of removing scripts from websites that make extensive use of them which can be beneficial on sites that use lots of cpu for example or load slowly because of those scripts.