This may be caused is one of the files that stores details of installed extensions is corrupt. If one of those files is corrupt it can cause problems installing/uninstalling add-ons and can stop them from working. First follow the procedure in the "Corrupt extension files" section of the [[Unable to install add-ons]] article to delete those files, then try uninstalling the Master Password+ add-on. If you are still having problems, try the procedures shown in the [[Cannot uninstall an add-on]] article.