Affected versions

• Firefox 66.0.3
• Firefox Beta 67.0b11
• Firefox Nightly 68.0a1

Affected platforms

• Windows 10 (x64)
• macOS 10.14
• Ubuntu 18.04 (x64)

Steps to reproduce

1. Start Firefox
2. Activate the sidebar, select "Synced Tabs" from the dropdown and then close the sidebar.
3. Open about:config, search for "identity.fxaccounts.enabled and set the pref to false.
4. Restart the browser and activate the sidebar.

Expected result

• The Synced Tabs category is disabled.

Actual result

• The Synced Tabs is still active.

Regression range

• This issue is not a regression. It is reproducible with the build (2018-02-27), where the pref was introduced.

Additional notes

• By clicking on "Sign in to Sync" after disabling the pref, will result in an empty page under about:preferences.

• The browser console's log can be seen in the image attached.

• On Firefox 66.0.3, by accessing the "Sign in to Sync" will redirect the user on the " about:preferences?entrypoint=tabs-sidebar#general ".

• Steps and behavior can be seen here.

:csasca, if you think that's a regression, then could you try to find a regression range in using for example mozregression?

I believe this is a minor defect - that preference is designed to be set by orgs etc who don't want sync to be offered at all, so it's very unlikely people will toggle it to false while they are signed in to sync - that's not a supported use-case for that pref.

And even if they do, the sidebar still showing seems fairly minor and difficult to fix - once they change the sidebar to anything else (eg, bookmarks), they become unable to change it back to synced tabs at all.