You probably happened to type something in a web page and instead of Enter you hit the Backspace key, losing all the data you typed while Firefox switches to the previous page.
Internet browsing has not changed much since the beginning of the Internet age, many of the shortcuts and buttons in modern browsers being similar to those of that time. With little exceptions.
Google and Microsoft have been eliminating one of the most used keyboard shortcuts for years, as many users end up losing typed data in web forms trying to delete text entered in boxes. It is obviously the Backspace key, which was universally used both to delete and to return to the previous page.
Inexplicably, Mozilla discussed 7 years ago the idea of eliminating the association between the Backspace key and the Back function, but only the rivals mentioned above actually made the change.
“To prevent the loss of user data when filling out forms, the Backspace key as a shortcut for the Go back one page function is now disabled. To reactivate this function you can manually change the browser.backspace_action parameter to 0, from the about: config menu. Alternatively, you can use the recommended shortcut: Alt + Left Arrow ”.
The change occurred starting with the Firefox Nightly 86.0a1 edition, and will be transposed into the official version of the Mozilla browser.