I've recently been experiencing the following problem when editing tables: using the visual editor in Firefox, if you double-click a cell with the intention to edit it, a new cell is added to the right and the contents of the cell you want to edit 'jumps' to that cell. The new cell isn't 'real' though: when you save the edit or preview the changes, everything looks fine. Nevertheless, it takes away the benefit of editing a table in the visual editor, because you don't 'get what you see'.
I made a gif to show what happens: https://gyazo.com/b740d94f3a34cf32dc7c4e01ffb94e43. And as you can see in this screenshot, when previewing the changes everything looks good: https://gyazo.com/737718a0ac661daf4a33112da659b113.
I tried it in safe mode, both logged in and logged out. In the Edge browser everything works just fine.