Mozilla has no problem with users who want to use a 3rd party application, but 3rd party apps need to 'follow the rules' to remain compatible with Firefox. At that point the end user gets frustrated with their paid-for application which makes itself dependent upon working with Firefox.

IMO, too damn many add-ons never seem to follow Firefox development in the alpha & beta pre-release testing months which is where the of the 'next' version of Firefox are made available (daily or twice a day, sometimes) for 3rd party testing and then find themselves behind the 8-ball when a new version is released to the end user. Perhaps OnePassword has 'some fixing to do', assuming that they use a side-loaded add-on as other 'security add-ons' have been doing. Create, store, and securely share logins, credit card information. Try out our browser extension beta and be among the first to use 1Password’s newest features. Mozilla does not make it hard, but beginning with Firefox the 'rules' for side-loaded add-ons was changed. 1Password is the world’s most-loved password manager, trusted by millions of happy customers and over 100,000 businesses. One Password is responsible for making sure that their Firefox add-on / 'plugin' is compatible with Firefox.

Firefox onepassword