Are you referring to the message bar that appears in this screenshot?

If so, this is a message from the Heartbeat feature. It's experiencing a bug right now. Best to disable it so you can get some immediate relief.

To disable Heartbeat: Open about:config in a tab then search at the top for browser.selfsupport.url and double-click it & set it to a blank value.

You may need to restart Firefox for the change to take effect.

Hey folks. The bug has been fixed in Heartbeat and you should not see the prompt anymore. While I can understand that it was annoying, I would not suggest disabling the browser.selfsupport.url feature permanently.

Heartbeat is the main conduit we have for communicating with our users. Based on the responses we get to our ongoing Heartbeat research programs we are changing the future of Firefox. We are committed to building products for our USERS. The only way for us to do this is to communicate with our awesome user base on a regular basis.

So please consider turning Heartbeat back on now that the issue has been fixed. If you see a Heartbeat prompt asking you for feedback, please considering replying. We want to hear from you!