8/2019 update: it looks like support for this, once tremendously useful, extension has lapsed.

After the recent update to the latest Firefox version, the browser kept on logging me out of all Google accounts every few seconds - which very nearly drove me nuts. This problem was not replicated on Chrome, so, back on Firefox, I disabled X-Notifier and the issue disappeared immediately.

If you are experiencing a similar problem, I suggest that you disable X-Notifier and see whether it persists. I did try reporting a bug, but the linked Forum and Twitter account haven't been updated since 2017. So I've now completely removed X-Notifier and won't be using it again.
[2017]
The change in layout I can live with. But having to re-enter ALL data for all e-mail accounts every time either the add-on or Firefox installs a new update is a real pain. Already looking for an alternative. Shame, because it was a really useful add-on.