Firefox asked for upgrading to 15.0 (from 14.0.1) whereas "do not check for updates" is checked.
Is this is a bug in the default configuration of Firefox or a bug on the Firefox side?
On previous Firefox versions, there were not such check, and only the package manager inform about updates.
I'm hoping this is fixed by adding
app.update.channel=false
in salix.properties as you suggested. A brief test shows that it is. Reopen if it isn't (we'll see in the next update).