Hi.

It's difficult to assess.

What computer OS versions , other apps, are you synced with another machine, etc.

So when you say disappeared, clearly not completely else how would Firefox know to request a re-install. Your browsing habits, it's pretty endless unless someone has a similar experience

Thank you Amoun. I am on Windows 10,and the machine is only synchedwith my 'phone. As you say, the fact that it asked to be reinstalled is a good sign, BUT Thunderbird completely disappeared fom the desktop last week. It was present on the disk, but smething was missing, as a simple copy did not solved the problem. I had to restore from backup.