How is that related to Firefox support?

It doesn't happen in Chrome or IE.

Aha, not Microsoft Excel, the program which is part of Microsoft Office, but Excel Online, which is part of Office 365.

I haven't noticed that behavior myself. Are you saying this happens when you open a sheet, when Excel selects, for example, cell A1 automatically, or that it happens in any cell you click?

Is the Undo arrow enabled on the ribbon (left end of the Home section) and, if so, can you undo the deletion that way?

If Excel Online doesn't seem to think there is anything to undo, does the content return to the selected cell if you click a blank cell, or is it gone permanently? In other words, is this a problem of broken display or lost data?

I don't know whether it's Office 365. I don't have that but i do use Excel online in OneDrive, the MS cloud app.

It happens in whatever cell is selected automatically AND in any cell i select myself.

The undo arrow is enabled but it doesn't work for the cell(s) selected automatically when Excel is opened. It works for the cells i select, but if i don't move to an empty cell with 4-5 seconds it deletes the content again. The content is gone permanently if i just move to an empty cell without using the undo arrow to restore the content.

Hmm, that's bad. I'm not sure what could cause that.

Could you test in Firefox's Safe Mode? In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any difference? (More info: Diagnose Firefox issues using Troubleshoot Mode)