As shown in the image here:
In French (and maybe other languages as well), the labels of the links (save/delete/cancel) are too long and the tooltip is pushed to the next line, where it makes very hard and sometimes to click on the "Add a qualifier" link (the whole line is unclickable, not only the part with the picture...)
A temporary workaround is to hide the tooltip alltogether in https://wikidata.org/wiki/Special:MyPage/common.css:
.mw-help-field-hint.wikibase-toolbar-item.wikibase-wbtooltip{ display:none; }
But it would be better that the bug doen't appear in the first place.
Bug seen in Firefox 48.0 on Ubuntu 16.10, with my regular account and another one without any personalizations.