I also have the settings checkboxes not working issue in Firefox. The POST request is sent and the settings do actually apply, it just looks like a caching issue with the returned page.
After enabling both Alert Notifications and Email Notifications and reloading the page (the previous state was alerts on, emails off):
In Firefox: (Missing checked attribute from Email
<div class="custom-control custom-checkbox mb-2">
<input name="notify_alert" type="checkbox" class="custom-control-input" id="checkNotifyFollower" checked>
<label class="custom-control-label" for="checkNotifyFollower">Alerts notifications</label>
</div>
<div class="custom-control custom-checkbox">
<input name="notify_email" type="checkbox" class="custom-control-input" id="checkNotifyContent" >
<label class="custom-control-label" for="checkNotifyContent">Email notifications</label>
</div>
In Chrome: (Correctly updated)
<div class="custom-control custom-checkbox mb-2">
<input name="notify_alert" type="checkbox" class="custom-control-input" id="checkNotifyFollower" checked>
<label class="custom-control-label" for="checkNotifyFollower">Alerts notifications</label>
</div>
<div class="custom-control custom-checkbox">
<input name="notify_email" type="checkbox" class="custom-control-input" id="checkNotifyContent" checked>
<label class="custom-control-label" for="checkNotifyContent">Email notifications</label>
</div>