Enabling multi-account container sync resets sync server settings

[alessandroberna]

Before submitting a bug report

• I updated to the latest version of Multi-Account Container and tested if I can reproduce the issue
• I searched for existing reports to see if it hasn't already been reported

Step to reproduce

• configure a custom sync server by setting identity.sync.tokenserver.uri to a custom value in about:config
• install the multi-account-containers extension
• enable syncing containers by clicking "start syncing"
  

Actual behavior

• The user will be prompted to log in their Firefox account even if they were already logged in.
• The identity.sync.tokenserver.uri will be changed back to https://token.services.mozilla.com/1.0/sync/1.5, completing the login process will trigger the browser to sync with mozilla's syncstorage service.
• changing the identity.sync.tokenserver.uri back to the custom value will "break" the extension, with this only displaying a blank page
  

Expected behavior

I would've expected this to sync with my own server, or at the very least display a warning/an error saying that using container sync with a custom endpoint isn't currently supported.
I think that silently editing that value may not be the best approach.

Additional informations

The extension is unaffected by the custom sync endpoint and works flawlessly as long as container sync is disabled. I’ve attached about:support data for two different browsers, as I initially encountered this issue on the Zen browser, a Firefox fork. Although it seemed unlikely that the bug was specific to the forked browser, I tested it on vanilla Firefox too and successfully replicated it.

Provide a copy of Troubleshooting Information page (optional)

https://pastebin.com/dHNLPwfr

[influential-eliot]

Could this be tied to the reason why containers seem to only sync at a profile creation?