[Bug 1858333] [NEW] Firefox sync not working, error in _fetchTokenForUser

[antgel]

Public bug reported:

Following info from about:sync-log

```
1578230581042	Sync.LogManager	DEBUG	Flushing file log
1578230581043	FirefoxAccounts	DEBUG	FxAccountsProfileClient: Requested profile
1578230581043	FirefoxAccounts	DEBUG	getOAuthToken enter
1578230581047	FirefoxAccounts	TRACE	getCachedToken returning cached token
1578230581047	FirefoxAccounts	DEBUG	getOAuthToken returning a cached token
1578230581049	Services.Common.RESTRequest	DEBUG	GET request to https://profile.accounts.firefox.com/v1/profile
1578230581057	Sync.LogManager	DEBUG	Log cleanup threshold time: 1577366581057
1578230581075	Sync.LogManager	DEBUG	Done deleting files.
1578230581790	Services.Common.RESTRequest	DEBUG	GET https://profile.accounts.firefox.com/v1/profile 304
1578231744394	FirefoxAccounts	INFO	Polling device commands.
1578231744395	Sync.Service	DEBUG	User-Agent: Firefox/71.0 (Linux x86_64) FxSync/1.73.0.20191205184924.desktop
1578231744395	Sync.Service	INFO	Starting sync at 2020-01-05 15:42:24 in browser session h18F_qHAu8gm
1578231744395	Sync.Service	DEBUG	In sync: should login.
1578231744395	Sync.Service	INFO	User logged in successfully - verifying login.
1578231744396	Sync.BrowserIDManager	DEBUG	unlockAndVerifyAuthState already has (or can fetch) sync keys
1578231744396	Sync.Status	DEBUG	Status.login: error.login.reason.network => success.status_ok
1578231744396	Sync.Status	DEBUG	Status.service: error.login.failed => error.login.failed
1578231744396	Sync.Service	DEBUG	Fetching unlocked auth state returned success.status_ok
1578231744396	Sync.BrowserIDManager	INFO	Getting keys
1578231744396	FirefoxAccounts	INFO	Migrating kB to derived keys.
1578231744401	Services.Common.RESTRequest	DEBUG	GET request to https://api.accounts.firefox.com/v1/account/device/commands?index=0
1578231744402	Sync.BrowserIDManager	ERROR	Non-authentication error in _fetchTokenForUser: Error: Can't change uid(resource://gre/modules/FxAccountsStorage.jsm:227:13) JS Stack trace: updateAccountData at FxAccountsStorage.jsm:227:13
1578231744402	Sync.Status	DEBUG	Status.login: success.status_ok => error.login.reason.network
1578231744402	Sync.Status	DEBUG	Status.service: error.login.failed => error.login.failed
1578231744402	Sync.BrowserIDManager	INFO	Failed to fetch the cluster URL: Error: Can't change uid(resource://gre/modules/FxAccountsStorage.jsm:227:13) JS Stack trace: updateAccountData at FxAccountsStorage.jsm:227:13
1578231744403	Sync.Service	DEBUG	verifyLogin failed: Error: Can't change uid(resource://gre/modules/FxAccountsStorage.jsm:227:13) JS Stack trace: updateAccountData at FxAccountsStorage.jsm:227:13
1578231744403	Sync.Status	DEBUG	Status.login: error.login.reason.network => error.login.reason.network
1578231744403	Sync.Status	DEBUG	Status.service: error.login.failed => error.login.failed
1578231744403	Sync.ErrorHandler	ERROR	Sync encountered a login error
1578231744403	Sync.SyncScheduler	DEBUG	Clearing sync triggers and the global score.
1578231744404	Sync.SyncScheduler	DEBUG	Next sync in 3600000 ms. (why=schedule)
1578231744405	Sync.Service	DEBUG	Exception calling WrappedLock: Error: Login failed: error.login.reason.network(resource://services-sync/service.js:1041:15) JS Stack trace: onNotify at service.js:1041:15
1578231744405	Sync.Service	DEBUG	Not syncing: login returned false.
```

```
15:51 $ lsb_release -rd
Description:    Ubuntu 18.04.3 LTS
Release:        18.04
```

```
15:54 $ apt-cache policy firefox
firefox:
  Installed: 71.0+build5-0ubuntu0.18.04.1
  Candidate: 71.0+build5-0ubuntu0.18.04.1
  Version table:
 *** 71.0+build5-0ubuntu0.18.04.1 500
        500 http://il.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages
        100 /var/lib/dpkg/status
     59.0.2+build1-0ubuntu1 500
        500 http://il.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
```

** Affects: firefox (Ubuntu)
     Importance: Undecided
         Status: New

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Title:
  Firefox sync not working, error in _fetchTokenForUser

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