When clicking on "Merge with" to use the merge tool on Wikidata with Firefox, nothing happens and we get this error in the web console:
```
Uncaught DOMException: Permission denied to access property "ownerDocument" on cross-origin object load.php:179
```
With debug=true:
```
Uncaught DOMException: Permission denied to access property "ownerDocument" on cross-origin object Element.js:301
getDocument Element.js:301
getDimensions Element.js:462
getSize Window.js:216
getSizeProperties Window.js:236
fitLabel ProcessDialog.js:263
emit jQuery
setLabel LabelElement.js:171
getSetupProcess Dialog.js:233
proceed Process.js:62
execute Process.js:97
setup Window.js:621
openWindow WindowManager.js:436
Uncaught DOMException: Permission denied to access property "ownerDocument" on cross-origin object Element.js:301
getDocument Element.js:301
getDimensions Element.js:462
getSize Window.js:216
updateWindowSize WindowManager.js:707
updateSize Window.js:454
setup Window.js:622
jQuery 2
```
It may be linked to T254693.