While this works mostly on a psychological level, prominently displaying identifying information such as the user s name/email address or a company name/logo, may also help prevent users from distributing your work because it is obviously personalized/tailored software, and they may not want this information to be distributed along with your software.
Also, once you do distribute your extension to your target users, you can digitally sign the XPI files for each individual user (i.e. fingerprinting individual files within the XPI package), so that you can track back any leaks.
In addition, you as the author of the extension are of course free to implement a simple "talkback" mechanism so that you can track use of your extension, along with all sorts of other information that may be relevant to you (i.e. usage stats).
Similarly, XPI files are conventional ZIP files, so you can also password-protect them to make it more complicated to install them without proper instructions.