Hello

did you restart Thunderbird ?

Hello

Yes, when I make the change for Sunday i restart it to take effect then few days or so later when I notice it, it goes back to Monday then have to repeat to get it back to Sunday.

As far as I know this preference, that you can see in configuration editor (calendar.week.start), is persisted in prefs.js. Could this file be touched by something else than Thunderbird (maybe antivirus) ?

No, I'm only using Linux and i don't have any antivirus running in background.

Only when i want to scan something

Oh well, you are right. Worse is that this behaviour seems to be by design. The first day in week is reset by some code in the bowels in the system, from a default that I can't fathom where it's coming from, maybe from the operating system itself ? I don't know. What is sure is that the 'user preference' is destroyed regularly.

Hi,

No other settings change, only the date in calendar for start of week else everything works perfectly.

Well yes, because what happens is not an accident, it's done quite on purpose. For some reason the developer has decided to override the user preference at the Thunderbird level by some other external preference, possibly set at the operating system level.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1800403

@sfhowes: needless to say, I can repro it with my locale (Thunderbird setup in US english, OS in French locale)