UPDATED REVIEW:

I originally wrote the review for addon v1.1.8. In my original review, I mistakenly thought that the addon's source code was only available by extracting the xpi file as "homepage" and "support" links lead to a different (issue-tracking only) repository than the code.

Since the author was kind enough to reply quickly and not only provide the link to the source code (https://github.com/igorlogius/toolbar-countdown) but to also build a new version with notificaiton support and add a demo video link to both the README file on github and the addon's description here on AMO. Since AMO didn't have any link for me to create a reply and say thanks, I thought I should at least retest with the new version and update my review. I retested the same day with FF 124.0.1 / addon v1.1.9 / Cinnamon desktop on Fedora Linux.

In my opinion, this addon is probably best suited to those looking for a simple, no-frills addon that doesn't require much in the way of permissions. If you are looking for lots of bells and whistles, you won't find that here (if so, the 2nd to last bullet under "CONS" might work for you).

Also the "CONS" aren't so much me trying to be critical or saying the addon needs something, more me trying to note what is or is not there for other users.

PROS:

- Open-source under BSD license with a public code repository (now linked to via addon description: https://github.com/igorlogius/toolbar-countdown).
- Simple design / not much to get confused over
- Can add a label for your countdown
- Tab + up/down arrows can be used to help fill out the form
- As far as I can tell, there are no arbitrary restrictions as long as the time occurs in the future (a different addon I tested would not allow countdowns of less than 1 minute - this addon has no such limitation).
- No permissions required unless you opt to enable notifications.
- Now supports notifications (to enable, go to Addons page and click the three-dots for this addon, select Manage, then go to Permissions tab and enable the Notifications permission).
- In my testing, the notification was displayed using system notification and I was able to see it, even with Firefox minimized and not in focus.
- For those who prefer 12-hr time format, that's what this uses.
- While there is no "snooze" functionality for alarms, since there is only one countdown, it is very easy to re-edit.

CONS:

- Possibly users that jump right in might get confused like I was when testing v1.1.8 / before author clarified (in thinking that the calendar icon is for a date-selection widget). I don't blame the dev at all (was my mistake) but also can easily see others making the same incorrect assumption. Hopefully, the new demo video will prevent this somewhat.
- If you don't like manually entering the date, you might be disappointed that there is no way to skip entering the date (it will not auto-infer "today" if you just try to fill out the time) and that there no presets or other features like that.
- For those looking for multiple countdowns, per dev response, that is outside the scope of this addon. There are other addons that support this (even other open-source ones like LibreTimer by Nudin47 though it uses MPL2 instead of BSD licensing if that is a concern).
- There is not sound during when the countdown goes off. That is probably also by design. If that is something you want see previous bullet.

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Apparently, I type too much. Had to remove most of the orig review to fit under the character size limits. Didn't want to completely lose the history, so tried to summarize so that dev response still made sense in context.

1. I noted that only 1 timer is allowed
2. I noted some confusion regarding the date input and
3. that it appeared manually typing out the entire date was required.
4. I noted that I wasn't receiving any sound or any form of alert/notification/etc when timer reached zero
5. As noted above, I mistakenly thought addon did not have a public code repo. I was wrong.