FF is firefox
Do you use cloudflare?
If you dont use cloudflare then that is not the issue, so nevermind.
If you do use cloudflare then you need to turn off auto minify under the speed section.
Other than that youll will need to disable all 3rd party plugins and then test it, if it works then you will need to turn on one plugin at a time and test to see which plugin has the conflict.
You can also try turning on dev mode and see if anything shows as an error.
And then after that check to see if the console shows any errors when you try to move it.
Do you know how to look at the console?