What versions of Firefox/Thunderbird did you try
afaik Firefox from the main repo is 52 esr build against gtk2
Firefox from mozilla is build against gtk3
you can try Firefox 53 from the mozilla repo as I think that one is build with gtk3
http://download.opensuse.org/reposit...USE_Leap_42.2/
you'd need to add the mozilla repo and do a full vendor change to it as I think installing only the Firefox rpm might cause issues
ps I'm still on plasma 5.8 with Firefox 52 esr and have no issues