Hello, Firefox's pop-up blocker may have prevented the file upload window from opening. Please try adding Gmail to your list of allowed sites by following the steps below:

1. Click on the 3-bar menu button and choose Options.
2. Select the Content panel.
3. Click on the Exceptions button next to Block pop-up windows.
4. In the Allowed Sites window, type in mail.google.com in the address field, then click Allow to add it to the list.
5. Click Close to finish.

EDIT: try one more time before the above suggestion.

thank you

Google has fixed this issue on their side, so you should no longer experience this. You may have to logout and login again or reload pages and bypass the cache (Ctrl+Shift+R or Ctrl+F5).

• https://productforums.google.com/forum/#!topic/gmail/q0BxcGOa6pk

THANK YOU VERY MUCH! YOU DID IT!