Update: it looks like this change will only be visible to a fraction of users initially, so please plan any changes to be backward-compatible.
The search UI was retooled and it will launch with Firefox 34 (at least in the US region). This landed late in the Firefox 34 beta cycle since it is related to our switch of default search provider in certain regions, which was announced very recently.
Unfortunately, this is bound to break some add-ons and themes. If your add-on overlays any content in the search UI or modifies its behavior, we strongly recommend that you test it on the latest beta (only the US English version seems to have this change for now). If you have any compatibility updates related to this issue in the review queues, please let us know on our IRC channel (#amo-editors) or the amo-editors mailing list.
Let us know in the comments if any of your add-ons break because of this.