Starting version 135, https://bugzilla.mozilla.org/show_bug.cgi?id=1934069 "show missed reminders" is off by default. You can change it in Settings.

Why would Thunderbird change something IMPORTANT like this?!!! I missed a reminder on Saturday and thought maybe I'd somehow accidentally dismissed it when Thunderbird came up. Today I realized a snoozed reminder had stopped popping up several days ago, and it's something I NEED to do within a couple of more days. I'd completely forgotten about it until I was adding another reminder for the same day. I've now changed the setting, so hopefully my reminders will work correctly again. I've also gone through my future events for the next couple of months and found some others which had early reminders which were snoozed and never reappeared. I hope I have fixed all of them now! I set up "events" to add important reminders so I can remember to take care of them. Some of these items are very important! DEVELOPERS, please take note and STOP changing things that could be CRITICAL to your users! If you want to add new settings, please keep the defaults the way they have always been! Thank you.

I've had this bug for years, however it isn't just when snoozed reminders occur during power off time.

Snoozed calendar events randomly fail to reappear.

Prior to my current 139.0.2 on archlinux these missing reminder popups could be recovered by shutting down thunderbird, and restarting. At which time a reminder popup would appear with all of the lost reminders. Now, once a snoozed reminder popup fails to appear, it never appears again.

This is a critical calendar bug. It is not new. I have filed a bug report for this before, but I find no record of it with a search. Maybe there was a purge of older unresolved bugs?

Calendar team, please address this! Thank you...

OK, changing the "Show missed reminders for writable calendars" to true, did cause my missing reminders to reoccur on restarting thunderbird.

Several notes:

1) I didn't get, and have never gotten, "bombarded by hundreds of missed reminders for past repeated events", as is stated in the above linked bug which led to the disabling of these reminders by default:

https://bugzilla.mozilla.org/show_bug.cgi?id=1934069

I would suspect that the "bombarded" reminders this user is experiencing are only the ones that calendar has previously failed to popup, i.e. lost. I use calendar reminders extensively, and have for many years, I've never had this barrage of reminders, but I have had the lost reminders for at least several years now.

2) Apparently reminders can be "missed" in more ways than by occurring during power off time. I rarely power off, and thunderbird is running continuously for extended periods. The main reason I restart thunderbird is to recover lost calendar reminders.

3) The link above, to the bug report leading to disabling by default, is malformed and only links to https://. This occurs in the reply:

https://support.mozilla.org/en-US/questions/1508947#answer-1729241

4) In this comment:

https://bugzilla.mozilla.org/show_bug.cgi?id=1934069#c4

it is stated: "We'll go through a substantial calendar rework this cycle so we'll fix all the past reminders issue with time."

I certainly hope this is the case. This is a critical calendar bug. If a user is relying on a calendar reminder for an important event, it can be a catastrophic failure to not receive that reminder (as the OP of this issue states.

I completely disagree with the statement in the same comment: "Let's go forward with this and disable this feature since it causes more problems than anything else."

Disabling reminders can be a catastrophic failure for a user depending on them. Disabling them by default was a bad idea IMHO.

I would suggest that the need to disable these reminders by default is completely due to other bugs in reminder behaviour. Primarily the accumulation of lost reminders that appear on thunderbird restart, and potentially on profile creation (although I don't recall ever having this experience then).

Once again, this is a critical calendar issue. Thank you to the calendar team for addressing the loss of event reminders!

Yes, I have noticed that occasionally when I snooze reminders a few times over the course of the day that they stop popping up until I restart Thunderbird. This has not been a real problem for me because I often set reminders for several days before something needs to be taken care of, and I keep snoozing them until I get it done. I normally power off my Macbook laptop every night; when I was using a Windows PC I often left it running but I closed all windows, including Thunderbird. So when the reminder popped up when I reopened Thunderbird the next day it wasn't a big deal because I probably wasn't going to take care of it the night before, anyway. I just use the 2-hour snooze because it's available in the drop-down and is easier to just click instead of resetting it for the next day. (FYI, I put the reminders on my calendar instead of using the "Tasks" function.). But it is a bug; the reminders should continue to pop up after being snoozed without having to restart Thunderbird.