I have this problem as well. It seems there is a conflict with my firefox settings and / or extensions, and some parts of the bitbucket features are not working correctly because of this.
For example I always got the pop up, which is annoying, and if I try to add a picture to my projects I can't (it just doesn't save). I couldn't also "vote" for this current issue. But if I log in with an other browser (chrome for example), everything is working fine, so I could press "got it" and it didn't appear again in firefox as well because it was correctly stored in my bitbucket settings.
I got very few similar problems with this firefox profile (maybe 2-3 websites, and bitbucket is one of them).
I found in my firefox about:config the network.http.sendRefererHeader option was set to 1, while by default it's set to 2. Using 1 was probably blocking some code, so turning it to 2 again was the solution to my problem.