Currently, there is a stable [https://download.kde.org/stable/KBibTeX/kbibtex-0.7.tar.xz.mirrorlist version for KDE4] (0.7, still KDE4-based), for which most Linux distributions include packages for easy installation. A [https://download.kde.org/unstable/KBibTeX/kbibtex-0.7.95.tar.xz.mirrorlist pre-release for the next stable release] (0.8-rc1, finally KDE Frameworks 5-based) is also available.
The [https://download.kde.org/stable/KBibTeX/0.8.1/kbibtex-0.8.1.tar.xz.mirrorlist stable version 0.8.1] is KDE Frameworks 5-based. In case you are still using KDE4, please use [https://download.kde.org/stable/KBibTeX/kbibtex-0.7.tar.xz.mirrorlist version 0.7].