Upon latest updates of Plasma 5.10, Firefox and thunderbird regularly crash when trying to open a file. For example, start TB, start write new message, add attachment (a window opens to select a file), click on the file to be attached and TB crashes. The very same with Firefox if you try to open a file and click the selected one. Everything ran smoothly before update. i.e. Plasma 5.10 beta.

In addition, I uninstalled both FF and TB provided by Leap 42.2 (via Yast) and replaced them by latest versions from Mozilla site http://download-origin.cdn.mozilla.net/pub/ . Now everything run smoothly. It looks like there is a problem with Plasma and integration of FF/TB thereto.

Hope this clarify the matter and the solution will soon be available

My system:
KDE Plasma 5.10.0; KDE Frameworks 5.34.0; Qt 5.9.0; Kernel 4.4.62-18.6-default; OS 64-bit