Does it only not work in a Profile Manager prompt when you load a website?

Are you possibly using Unicode characters in the MP?

I assume that it works in the Device Manager as well?

• Options/Preferences -> Privacy & Security
  Certificates: Security Devices > Software Security Device -> Login

@cor-el Thank you for your answer.

There is an accentuated character in my master password. It was there before the problem started.

The MP also works in the Device Manager, just not when I load a site and am prompted to enter my master password.

You can try a MP without accented characters beyond the 80-ff range to see if that works better.

@cor-el Thank you for your answer.

I tried setting a password with no accentuated character, and that worked. I then tried setting a password with the accentuated character again, and that worked too, but not always and I fail to understand why.

When the MP would not be accepted upon starting Firefox, it would work when entered in Settings > Privacy.

The worst now is that the behaviour is fluctuating. The fact that it doesn’t accept the accentuated MP several times (closing and reopening Firefox in between) doesn’t mean that it won’t finally accept it, and the MP is entered properly each time.

I run both Stable and Nightly on Ubuntu and both built react the same way.

It might be time to open to report a bug, what do you think?

The problem is now gone in Nightly 69.0~a1~hg20190703r481084-0ubuntu0.19.10.1~umd1, but present in stable 68.0+build1-0ubuntu. I seems to be a bug that has now been fiexed.

Does it always work if you use the Login button in Security Devices?

• Options/Preferences -> Privacy & Security
  Certificates: Security Devices -> Software Security Device

I'd day yes.