The images are I think self explanatory...

Hello Soumitra2102,

Thank you for contacting Mozilla Support. From the screenshots you've provided it looks like your plug-in is disabled. Does the right-click menu appear when you enable it? You may have to restart Firefox after enabling the plug-in.

In addition I did some poking around on the DAP support page and found this:

First open Firefox, click on Tools and choose Add ons. Find the Download Accelerator Plus Integration and verify that it is enabled. Then make sure that DAP is set up for integration with FireFox - Click the Tools menu in DAP's main window then click Options. Choose Advanced and make sure the Mozilla/FireFox Integration option is selected.

Please let us know if this solves your problem.

Cheers, Patrick