Coming in Firefox 65: Dedicated Profiles Per Install and Profile Downgrade Protection

Dave Townsend dtownsend at
Thu Oct 18 19:32:36 UTC 2018

In Firefox 65 we intend to ship two new features to help prevent user
frustration caused by using profiles created by newer versions of Firefox.


Firefox stores all of its settings in the user’s profile and unless certain
command line arguments are used Firefox always launches with the same
profile. Periodically as Firefox upgrades it makes changes to the settings
that can render the profile unusable by earlier versions of Firefox. This
can cause anything from certain features of Firefox being broken after a
downgrade to Firefox crashing on startup.

To protect against this two new features will be landing in Nightly soon.

Dedicated Profiles Per Install

A common cause of users switching between different versions of Firefox is
having Firefox installed multiple times. This most often happens when users
have different channels installed at the same time like ESR and Release. It
is a common request to be able to run the different installs at the same
time. In Firefox 35 this was made possible for the developer edition by
giving it a dedicated profile. The new dedicated profiles per install
feature extends this and will give every install of Firefox on the computer
a dedicated profile by default.

Users will be able to run different installs of Firefox simultaneously.
Each will use a different profile for its settings. Upgrading an install of
Firefox will keep it using the same settings.

We’re tracking work on this feature in bug 1474285.

Profile Downgrade Protection

For cases where users manually downgrade an install of Firefox or attempt
to forcefully use an older version of Firefox with a newer profile the
profile downgrade protection feature will now tell the user that the
profile is too new to use with this Firefox giving them the option to
create a new profile to use or to quit.

We’re tracking work on this feature in bug 1455707.

Developer Implications

As developers it is quite common to switch between different builds for
example when testing different built versions of Firefox and doing
regression testing. To support these use-cases a new command line argument
“--allow-downgrade” will allow skipping the downgrade protection.


While some users may be impacted by this change we believe that most users
will be unaffected and those that are will see less problems caused by
downgrades than previously. This will give us the ability to ignore the
possibility of downgrades when implementing new features and fixing bugs
going forwards. Being able to make the assumption that this code works as
designed will allow us flexibility in other changes downstream.

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