can't select partial text of a cell in firefox

Steps to Reproduce

1. import the attached file or enter a text abcdefgh at A1 with Firefox
2. double click A1 to enter edit mode
3. select only 3 characters of the text by drag-n-drop

Current Result

it always selects the whole text

Expected Result

allow to select partial text

Debug Information

• chrome doesn't have this problem, but it could its bug: https://stackoverflow.com/questions/37913629/chrome-appears-to-ignore-user-select-in-a-contenteditable-div

Workaround

[contenteditable]