I have debugged this issue a bit and found, that the runners/bookmarks/faviconfromblob.cpp file in plasma-workspace tries to access the table moz_favicons in the sqlite database (line 80) which doesn't exist though.
Checking the firefox source code, I found mentions that since Firefox 37 the table was not used anymore and since Firefox 41 the table was dropped. I have no idea why this wasn't an issue before but it might be because of a newer Qt version that broke something?


$ sqlite3 .cache/krunnerbookmarkrunnerfirefoxdbfile.sqlite                                                                                             
SQLite version 3.22.0 2018-01-22 18:45:57
Enter ".help" for usage hints.
sqlite> .tables
moz_anno_attributes    moz_historyvisits      moz_keywords         
moz_annos              moz_hosts              moz_places           
moz_bookmarks          moz_inputhistory     
moz_bookmarks_deleted  moz_items_annos


gdb backtrace (without Qt debug symbols):
(gdb) bt
#0  0x00007ffff41cba60 in QVariant::QVariant(QVariant const&) () at /usr/lib/libQt5Core.so.5                                                                                                                                                                                   
#1  0x00007fffb7d682cc in  () at /usr/lib/qt/plugins/sqldrivers/libqsqlite.so                                                                                                                                                                                                  
#2  0x00007fffb7d65ae6 in  () at /usr/lib/qt/plugins/sqldrivers/libqsqlite.so                                                                                                                                                                                                  
#3  0x00007fffeef07938 in QSqlQuery::exec() () at /usr/lib/libQt5Sql.so.5                                                                                                                                                                                                      
#4  0x00007fffb7f7c761 in FetchSqlite::query(QString const&, QMap<QString, QVariant>) (this=this@entry=0x555557af0420, sql=..., bindObjects=...) at /tmp/plasma-workspace/trunk/src/plasma-workspace-5.12.1/runners/bookmarks/fetchsqlite.cpp:80                               
#5  0x00007fffb7f7cf0a in FetchSqlite::query(BuildQuery*, QMap<QString, QVariant>) (this=this@entry=0x555557af0420, buildQuery=0x555557a6a0a0, bindObjects=...) at /tmp/plasma-workspace/trunk/src/plasma-workspace-5.12.1/runners/bookmarks/fetchsqlite.cpp:67                
#6  0x00007fffb7f77972 in FaviconFromBlob::iconFor(QString const&) (this=0x555557a776a0, url=...) at /tmp/plasma-workspace/trunk/src/plasma-workspace-5.12.1/runners/bookmarks/faviconfromblob.cpp:132                                                                         
#7  0x00007fffb7f751b5 in BookmarkMatch::asQueryMatch(Plasma::AbstractRunner*) (this=this@entry=0x7fff957f9c70, runner=runner@entry=0x555557a6a330) at /tmp/plasma-workspace/trunk/src/plasma-workspace-5.12.1/runners/bookmarks/bookmarkmatch.cpp:63                          
#8  0x00007fffb7f80064 in BookmarksRunner::match(Plasma::RunnerContext&) (this=0x555557a6a330, context=...) at /tmp/plasma-workspace/trunk/src/plasma-workspace-5.12.1/runners/bookmarks/bookmarksrunner.cpp:85                                                                
#9  0x00007fffd10887f0 in Plasma::AbstractRunner::performMatch(Plasma::RunnerContext&) (this=0x555557a6a330, localContext=...) at /tmp/krunner/trunk/src/krunner-5.43.0/src/abstractrunner.cpp:130                                                                             
#10 0x00007fffd0e67618 in ThreadWeaver::Executor::run(QSharedPointer<ThreadWeaver::JobInterface> const&, ThreadWeaver::Thread*) () at /usr/lib/libKF5ThreadWeaver.so.5                                                                                                         
#11 0x00007fffd0e66610 in ThreadWeaver::Job::execute(QSharedPointer<ThreadWeaver::JobInterface> const&, ThreadWeaver::Thread*) () at /usr/lib/libKF5ThreadWeaver.so.5                                                                                                          
#12 0x00007fffd0e65d45 in ThreadWeaver::Thread::run() () at /usr/lib/libKF5ThreadWeaver.so.5                                                                                                                                                                                                                                                                                               
#13 0x00007ffff3f98b4d in  () at /usr/lib/libQt5Core.so.5                                                                                                                                                                                                                                                                                                                                  
#14 0x00007ffff17f208c in start_thread () at /usr/lib/libpthread.so.0                                                                                                                                                                                                                                                                                                                      
#15 0x00007ffff38a0e7f in clone () at /usr/lib/libc.so.6 

If the backtrace from Qt is needed, I can recompile Qt with debug symbols.