There is a bug in Firefox which prevents a lot of extensions from working when placed in the overflow menu. Until https://bugzilla.mozilla.org/show_bug.cgi?id=1373490 is fixed, you should keep those extensions visible on the toolbar.
Interestingly, the bug behaves differently on different operating systems. So for those with Linux, the limit is just large enough to cause no problems; Windows users have a smaller limit (as you have both noticed); I can’t find out about Mac OSX.
Unfortunately there is no way that an extension with as much content to display and interact with as Kee does could ever fit everything into the limit that the Windows variant of this bug imposes on Firefox users so we can’t make any modifications to workaround the bug and thus must wait for Mozilla to fix it in some future version of Firefox.
Please feel free to keep Kee in the overflow area if you don’t need the features or information on the far right hand side (or like in the case of the button to add a new entry, you can remember what it is for) but I’d still recommend occasionally using it from the main toolbar, especially after a new version is released, just in case something important changes that you otherwise would not be aware of.