Yes I understand the situation, I'm still on FF 10 haven't done the ver 11 update yet. But yes they have had some quirks happening with some to the updates.
I can't seem to duplicate your specific problem, but one thing I did notice is I can just double click on the symbol box and it highlights the whole old symbol, then I just type in the new one. Of course I can still highlight it and hit delete, then type in the new one.
Maybe there are also some new mouse settings that are now a little different.
good luck
Quasi
News 
Market Data 
Discover 
AI
