The removal of the Backspace key as a navigational element didn't come out of the blue. It was first proposed back in July 2014, in a bug report opened on Mozilla's bug tracker.
At the time, Mozilla engineers argued that many users who press the Backspace key don't always mean to navigate to the previous page (the equivalent of pressing the Back button).
"Pressing backspace does different things depending on where the cursor is. If it's in a text input field, it deletes the character to the left. If it's not in a text input field, it's the same as hitting the back button," said Blair McBride, a senior software engineer for Mozilla at the time.
"Whether to keep this behaviour has been argued For A Very Long Time," McBride said. "It's confusing for many people, but we've assumed it would break muscle memory for many people."
Back in 2014, McBride asked other Mozilla engineers to gather data and see exactly how many people press this key before taking a decision.
Subsequent data showed that the Backspace key is, by far, the most pressed keyboard shortcut inside the Firefox user interface, with 40 million monthly active users pressing the key and triggering a "Back" navigation.
To put it in perspective, this was well above the 16 million Firefox users pressing the CTRL+F shortcut to search content inside a page and 15 million Firefox users who pressed the page reload shortcuts (F5 and CTRL+R).
This giant difference between the first and other popular shortcuts led Mozilla engineers to conclude that many of the Backspace key presses were most likely accidental — with users pressing the key thinking the cursor was focused inside a form or search field, but accidentally navigating back a page, most likely also losing form data as a result.
Chrome removed support for the Backspace key in 2016
These discussions about the role of the Backspace key inside Firefox also took place around the same time that Google engineers had similar talks about Backspace in Chrome.