Issue weight cannot be edited in firefox

Description of the problem

The weight of an issue cannot be edited anymore. If you click the "Edit" link, the input field for the weigt is briefly visible and then it immediately vanishes again.

System information:

• Windows 10 x64
• Firefox 62.0.3

Approximate date/time when the error occurred.

Some time today I think... it still worked yesterday (ie. less than 24 hours ago).