mike96 said

be sure this popup comes from FF and not from a virus

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Thank you!

Are you using sync, and are you using the Master Password option?

If you are, then when sync tries to access the Password Manager, the Master Password is needed.

A possible cause of the MP prompt at startup can be the hotfix extension to add the new intermediate certificate to the Firefox Certificate Manager. If you have this extension listed on the about:studies page then try to remove it when possible.

If you run Firefox 66.0.5 then you no longer need an extension or other workarounds to install the new certificate, so possibly also check the about:addons page.