I think it is rather a novice question but I am not able to find it anywhere on the Web.
Clicking on the Toolbar Button of some add-ons(ex: DuckDuckGo, Amazon Assistant, Cookie-auto delete),there is something like a “drop-down menu”. How to add that type of functionality in my add-on ?
I wanted to show you what that “drop-down menu” looks like but I think, this platform does not support images so here is the link: https://pasteboard.co/Hpvhltv.jpg
God shall furnish you with so much wealth that even Bill Gates+ Jeff Bezos will be jealous of you.
Thank you so much!
Wow, I didn’t realize it could be so rewarding to answer questions here. 
You don’t know!
I was getting mad about this problem. Believe me, this problem was hitting a newbie like me in the face.
Actually it does. Just drag- & drop the image into the browser. Or paste it…
As for the actual question: A different way would be to use the menu API to create a context menu when you right-click on a toolbar button.
I could not post directly.
I sent a mail to addons+amo@mozilla-community.org and then the bot on the other side must have posted it. When I tried to attach an image with the email, I received an email from mozilla that “your post could not be posted”.
It does not know how to handle attachments. It may work with inline images when sending an HTML response via e-mail.
I can try that next time.
Thanks a lot freaktechnik!!!
