I have setup "extensions.masterPasswordPlus.suppress" to "2" and, as the title says, all popup windows asking for the Master Password are showed up and even stack up. I haven't messed up with the configuration in "about:config" yet, and I don't intend to unless required.
Using:
- LUbuntu Linux 14.04, up to date.
- Firefox 45.0.1, up to date according to the repos.
- Master Password+ 1.21.6, up to date according to Firefox.