How to show dot symbol under the icon of the opened application when searching it using super key

Most probably you don't have a correct StartupWMClass entry in your manually created .desktop launcher for the application. To add one, follow the steps below.

1. Launch Firefox application in question.
2. Run xprop WM_CLASS in a Terminal window. The cursor should turn into a crosshair.
3. Place the cursor over the opened Firefox window and click. You should get a WM_CLASS string for the application in Terminal.

4. Open the manually created .desktop file in a text editor and add (or modify) the following line

   StartupWMClass=OBTAINED-VALUE
   

   In place of OBTAINED-VALUE put a value you got from step 3 without any quotes.

5. Save the .desktop file.