To reproduce the issue, just load this example:

Ext JS 7.0.0 Examples > Kitchen Sink > Grids > Add-Ons and Decorations > Grid Filtering

If you open it using google chrome, to open any of the grid header's dropdown menus, only one click is enough.

If you open it using Firefox 71, to open any of the grid header's dropdown menus, two clicks are necessary: one highlight the header cell, a second expands the menu itself.

EDIT: if you click right outside the boundary box of the arrow down glyph in the dropdown, it is expanded at once in Firefox; if the click happens within the arrow glyph boundary box, then the two clicks issue is reproduced.