You would need to create a CommonJS module. Add your .js file inside the addon's lib folder and export all functions that you will be needing via the "exports" directive. Once you do that, you can use the exported functions via the "require" directive.
For example in the module that you will be reusing, you can put:
// REUSABLE MODULE
exports.somefunction = somefunction;
function somefunction() {
doSomething();
}
And then in the module that will be using this:
var othermodule = require("reusable_module");
othermodule.somefunction();
Here is the relevant documentation: https://addons.mozilla.org/en-US/developers/docs/sdk/latest/dev-guide/guides/modules.html