Was it a Google calendar (or other web-based calendar) you viewed in Firefox?

Or was it a "Lightning" calendar for the Thunderbird email client?

I don't think it was a Google Calendar program. It came some time (years) ago with Mozilla Firefox. Its name may have included "Reminders". Originally, the icon of a ribbon showed up on the top of the browser, like next to "Help". You clicked on it there you got the Calendar. A few months ago, the ribbon icon disappeared and you had to click on "Tools" and that program showed up in about the 4th position. I think the data was stored on the Web, and therefore I was hoping to recover it.

Sorry, I don't know what Thunderbird is or Lightning.

Thanks for trying to help,

Joost Mortelmans

Perhaps it was an add-on? You can search the add-ons site by tag or text.