The feature to comment with a screenshot is only available with Google Chrome, but the ability to comment on screen should be available with all browsers, e.g. Firefox. If you're unable to view the "Comment on Screen" button in the Discuss tab of your Axure Share project, then please ensure that the RP file that was published to Axure Share was uploaded from RP 8 (build 3372). You can update by grabbing the installer from www.axure.com/update. If updating to RP 8 doesn't help, however, then please ensure that discussions have not been disabled for the prototype. You can check this by browsing to share.axure.com in your browser, clicking on the gear icon under "Configure" for the project to select "Discussions", and clicking on the "Enable Discussions" button:
If that doesn't resolve the issue, would you be able to post your RP file and the project URL here for me to test? If you'd rather keep those private, please email firstname.lastname@example.org, and someone from our support team will be happy to further troubleshoot from there. Ty!