LemniscateOfRevolution said

"Do you want to change your default web browser to “Safari” or keep using “Firefox”?"

That is not from Firefox. It might be from your protection programs.

Firefox asks for master password each time it is opened

Did you set a Master Password? Did you want it turned on?

- Don't have a protection program... dialog was from the OS. - I have had a master password forever, and I want to have one. More: - Did a clean install Tues. of OS X 10.13. Restored my files from backup. - Noticed master password issue. Decided to investigate. - Repeated clean install and file restore. - Tested Firefox (which was set as default). All good. No master password issue. - Started safari and was browsing with it when dialog appeared, posted apparently by OS. - Before interacting with the dialog at all, reopened Firefox. - Firefox now asks for master password every time it is opened.

Are you using sync?

I was told this is not current, but; There is a preference that sets how long the Master Password is unlocked:

http://kb.mozillazine.org/About:config_Entries#Security. Search for; security. ask_for_password and; security.password_lifetime

In case the above doesn't solve the problem :

This could be a "souvenir" from the latest hotfix for "the issue" that disabled all extensions some days ago. If your extensions are (no longer) disabled, you could try to remove hotfix-update-xpi-signing-intermediate-bug from "about:studies".

Removing the study fixed the problem. Thanks!

LemniscateOfRevolution said

Removing the study fixed the problem. Thanks!

My pleasure - I'm glad everything is back to normal  !