As it is well known, WebExtensions manifest v3 disallows the execution of "remote scripts", so all extension logic should be packed within the extension. Although this measure may strengthen the security of the add-on ecosystem, it completely eliminates the possibility to customize add-ons with the user-provided logic written in JavaScript. Such scripts are stored in the extension local storage and are not remote by any means. This ability may be important for the workflow of some existing extensions. There may be many possible solutions of this problem, and one of them is to add an about:config switch enabling use of eval in MV3 add-ons for people who know what they are doing.
Please keep Firefox a browser that is open to innovation and invention. With the current approach to MV3 in Firefox, the only solution of the problem of the add-on customization for an end user is switching to an another browser that allows it.