User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:77.0) Gecko/20100101 Firefox/77.0
Steps to reproduce:
Windows 10 running in tablet mode, with the taskbar 'auto-hide' setting enabled.
'Tap' any text entry field including but not limited to the address bar.
The same is the case for Firefox Nightly 79.0a1 (2020-06-12) (64-bit).
I am running a Lenovo L390 Yoga (2-in-1 laptop with wacom layer, and touchscreen) with up-to date Windows 10 build 2004, all Lenovo software/firmware up to date.
The onscreen keyboard appears, and then automatically dismisses itself as soon as the animation to appear is finished. You cannot make the keyboard reappear until you 'tap' out of the text field, and then back in it again, when the onscreen keyboard once again exhibits the same behavior continuously and consistently.
The onscreen keyboard appears, and persists, until it is manually dismissed, the way it behaves in the new chromium based Microsoft Edge under the same circumstances.