Comment 2 for bug 1579587

The same issue, and it's not Ubuntu-specific.

The reason is that Firefox does not write `intl.accept_languages`
value to the new profile's `prefs.js`.

After creating a new profile, the first launch is nice and dandy:
websites are in Russian, as I expect. But after I close Firefox
and open it again, they turn English.

The language priority is correct in GUI, but it's missing from `prefs.js`.
To fix this behavior, I go to GUI settings, put the first language down,
then put it back to top and click "OK". Just clicking the button
without changing the order doesn't help.

To reproduce the bug, create a new Firefox profile, launch Firefox with it,
*exit and launch again*. Then go to e.g. https://www.debian.org/
It should be in English instead of your system language.
Check the profile's `prefs.js` to make sure there's no
`intl.accept_languages` in it.

Now do the above priority change thing and refresh the page. It should be
in your language now. Check the `prefs.js` again, the setting
should be there.